6 explanations for Fermi's Paradox

This is a list of numerous explanations for "Fermi's Paradox": The question of why we have not found a rapidly-expanding alien civilisation, which we would expect to take over each colonised galaxy in a matter of millions of years and reach neighbouring galaxies over a similar timescale. Such a civilisation would take over the galactic neighbourhood in what is merely a blip in Earth's timescale, so as we have not noticed the presumably obvious signs of one, either no alien civilisation exists amongst the trillions of stars in our galactic neighbourhood or if there is one it behaves differently to how we would expect.

PLANETS AND STAR SYSTEMS ARE MORE DIVERSE THAN WE THINK
It is often assumed that all that is required for a planet to give rise to life is a reasonable temperature, a CO2 atmosphere, a hot core producing volcanism and a magnetic field, and some water.
However, it assumes most planets are like the ones in our solar system, when in fact most exoplanets may be disqualifying themselves from hosting life even in the rare event they have all these traits.
Every star system is created from different mixes of elements, each planet receives portions of that mix in different ratios, and slightly different mixes of elements can produce vastly different results since a small difference in the ratio of two interacting elements might be all it takes to upset the balance so that huge quantities of one of those elements are available to produce a very exotic environment.
The surface of a planet represents only a tiny portion of the composition of that planet, so only a small change in the composition of that planet is necessary to completely change the surface. So a star system with slightly more carbon might be full of planets coated in carbon compounds, a star system with less oxygen could have huge amounts of pure metal creating strange geology and chemistry, a star system with more fluorine could have chemically inert planets, and all sorts of other stuff could happen.
So it's very possible that most planets are so inhospitable, that merely being a rocky body with Earthlike chemistry is rare.

EARTH IS MORE THAN JUST HOSPITABLE
Earth has a number of things that are rare or even unseen for other planets: Those things are an influential moon, an axial tilt, and moving continents. These features play two key roles in the development of life.
Firstly, they greatly diversify the conditions that are and will be present on Earth, which can drastically increase the chances that one places so happens to evolve life. The moon increases and decreases water pressure and changes currents, seasons change the temperature, and moving continents create a huge range of changes in conditions as well as creating volcanic features. All these constantly varying conditions create a much better chance that at some place, at some time, the conditions are just right for life to first develop.
Secondly, they stimulate evolution. Evolutionary simulations always seem to stagnate, and this is because they incentivise a species to evolve until they take on a form which all mutations of are inferior for surviving the conditions of the simulation. Varying conditions present new challenges for evolution to overcome, preventing evolution from ever ending. A A group of aquatic creatures may find themselves stranded as the moon causes the water to recede, all dying but those that can survive for a decent while in air. A merging of continents forces creatures to adapt to creatures that they have never seen before. A change in season requires creatures to adopt new behaviours. Changes in season can also create much greater and longer lasting effects; since weather is prone to feedback loops, the temperature swing from a season may be amplified more and more each year, completely changing the climate of huge areas (forest can become savannah, which is a theory for how homo erectus evolved) or even the entire world.
Earth isn't just hospitable, it's perfect for seeding life and stimulating evolution. The chances of a planet having both the hospitality of Earth and all the life-evolving traits of Earth are extremely slim, given that we haven't seen a single other planet with either quality let alone both.
It should, however, be noted that an Earthlike body in close orbit of a gas giant automatically has tides, seasons, and geological activity. The gravitational influence of the gas giant creates tides (assuming liquids for tides to affect) and tidal heating (by extension geological activity), and the gas giant can reflect or obscure the sun creating short (possibly even shorter than a day on the moon) seasonal cycles as the gas giant either lights up the night or hides the day. Thus for a moon, the life-evolving traits of Earth are not too unlikely.

SMART ANIMALS ARE UNLIKELY
Increasing the intelligence of a species makes the species more delicate and energy-consuming. The only thing it is useful for is allowing the species to survive where instinct fails it. Thus, the vast majority of species are extremely stupid and are guided by pure instinct (both genetic and that learned by linking stimuli and observation), it is almost always better as instinct is usually enough to cover everything the animal will encounter. The most intelligent species on Earth are thus those that must understand so much that sapience is the only possible way of understanding it all, which is why the only animals that appear to be sapient are living in diverse rainforests (some primates and parrots), travelling vast distances over land (humans and elephants), or travelling all over the world's oceans (dolphins and predatory whales).
Over the Earth's entire history, only a handful of sapient species able to demonstrate thought outside of instinct appear to have developed. All modern examples of sapience are birds and mammals, which didn't exist as we know them until recently, and sapience seems to be an effective enough survival tool that a species using it effectively would rarely go extinct naturally. So it can be presumed that the only sapient animals to ever exist have been around in human times, maybe a few more that were unlucky exceptions to the survivability of sapient species.
So while humans are just a blip in Earth's history, sapience of any kind hasn't been around much longer, a bloop to our blip. Given that life had been evolving for billions of years before sapience, but was expected to end (besides simple organisms) a mere billion years after due to expansion of the sun, there was a very real chance that even on Earth; an ideal world for advancing evolution due to ever-changing conditions; life would begin and end without a single intelligent thought being formed.
What this suggests is that no matter how bountiful a planet is with life, if it doesn't have great ecological diversity as well as species that wander thousands of kilometres (shorter distances are acceptable if ecological diversity is greater), no level of sapience will occur let alone a galaxy-conquering alien civilisation.

LIFE IS NATURALLY REGRESSIVE
When civilisation forms, there's no guarantees that it will eventually conquer the galaxy; in fact, there's something closer to a guarantee that it won't.
As sure as gravity, there is a phenomenon that pulls life down the evolutionary ladder, and that is evolutionary selfishness.
Not intentional selfishness per-se; "evolutionary selfishness" in this context means "anything bad for the evolutionary success of a species that contributes to the evolutionary success of an individual". Evolutionary selfishness is observed in any species that relies on groups; for example, pack hunters typically have a very strong individual in the pack that eats more than its fair share of food and has more than its fair share of children, while contributing equally or less to hunts.
Humans are no exception, and evolutionary selfishness takes the form of an inverse correlation between contribution to humanity and the number of children raised.
At one extreme are "savants", excellent independent thinkers with little intuition for people and emotions. They almost never have children (who would also likely be savants), but given the opportunity they can contribute significantly to whatever field piques their interest.
At the other extreme are male rapists who instinctively want to impregnate women, and women who trick men into impregnating them to collect child support and welfare.
Obviously, there's a massive difference between the evolutionary success of these two groups: One almost never has children and focuses on science, and the other revolves around having children. So you'll almost never find a savant with children as they'll be too focused on their work, but you can find generation after generation of people with no ambition but to rape women or exploit their children.
This is not a purely genetic phenomenon, it is also memetic, i.e. it depends on education and culture; but those things will be mostly the same between a child and their parents, so it behaves like a genetic phenomenon.
This phenomenon can be called a combination of dysgenics (the genetic accumulation of evolutionarily selfish behaviours) and dysmemics (evolutionarily selfish behaviours being passed non-genetically from parents to children). Dysmemics are far more pronounced than dysgenics, particularly in third-world countries, but fortunately can be prevented by making sure a good education is available to everone. The battle between education and dysmemics will shape humanity in the 21st century, as runaway population growth is currently happening in countries with poor education, and the impoverished population of Earth will rise by several billion by 2100 if this is not dealt with.
Dysgenics is much slower acting, and is difficult to observe as it is always paired with much more pronounced dysmemics. It is of course much harder to deal with since eugenics (also known as genocide, a bad idea for building a civilisation) is the only manual way to counteract it on a planetary scale, but natural processes can also deal with it: When survival becomes tough (which might be because of dysgenics/dysmemics eroding society), the accumulated effects of centuries of dysgenics are quickly eliminated.
Dysgenics may be a weak enough phenomenon that humans, and other possible alien civilisations, do not have to worry about it.
Dysmemics, however, poses a major threat to any civilisation that has both thriving and non-thriving parts and requires active measures to counter. Any civilisation that does not understand and thus counteract dysmemics will peak quickly then slowly degrade.

DESTROYING THE WORLD IS EASY
One of the most common explanations for Fermi's Paradox is that aliens inevitably invent nuclear weapons then destroy themselves. However, to use humanity as an example, we should be able to survive conventional nuclear warfare with small bunkers or even just living far from the war zone, and the worst case scenario of a global bombardment of salted nukes could be survived by small islands that have bunkers and would quickly lose contamination to the ocean.
Nukes may not be dangerous for a planet-wide civilisation where the populace knows how to survive, but there are other things that aliens are likely to invent that might wipe them out.
Something that consistently destroys alien civilisations would be an obvious enough that all aliens think of it, and easy enough to create that at the time of creating the apocalypse the civilisation is no bigger than the area of devastation. The most obvious potentially-apocalyptic invention is artificial life: It's a very common idea amongst humans (most religions consider us artificial life), we can probably create it before we permanently settle other worlds, and the area of devastation would be our world.
Artificial life, even AI, will not be a homogenous and systematic force of destruction, at least not for very long. Evolutionary selfishness guarantees anything with the capability to evolve will experience dysgenics if it is too successful. However, the ecological shock of a radical new species being introduced to an ecosystem is devastating, so the ecological shock of even a non-malevolent artificial life form could be enough to wipe out most species on a planet; likely including the inhabitants of that planet, since the new life form may wipe out their food source or even them.
Since this explanation for Fermi's Paradox requires that the destructive invention is obvious enough that everyone invents it, the destructive invention can't be anything we haven't thought of yet. So, if aliens keep on destroying themselves, it's because of artificial life, since it's the only thing we've thought of that we might not be able to survive.

EVOLUTION DOESN'T FAVOUR EXPERIMENTAL EXPLORATION
Obviously, a species that can spread more easily has an evolutionary advantage. Birds can be found on every hospitable landmass, trees find ways to get their seeds to travel further, and there are spiders that have learned to fly. Thus it's easy to assume that evolution eventually leads to space travel.
But consider a bird that can only fly a hundred metres at a time: It can't reach another landmass yet, so it receives no advantage in spreading from the ability to fly.
As humans, we are like that bird that can only fly a hundred metres: We can go into space, but we can't yet live on other planets. What keeps us going is not evolution, only curiousity and ambition, the "things that make us human".
So what about aliens? Will they put aside what mother nature calls them to do, dedicating enormous amounts of time and energy into something that may take a century to yield even the slightest evolutionary reward? Perhaps not, perhaps they will only ever have short-sighted goals in mind. Even humans, a species so forward-thinking that many specimens worry about what will happen a billion years from now, still has almost the entire population focusing on short-term goals. Even the wealthy, the only people with enough money to advance science and engineering, are all focused on obtaining more money. So anything less curious and ambitious than humans stands no chance of making it to sace.

These are 6 different explanations for Fermi's Paradox.
In reality, none of them should be strong enough to explain why we haven't seen aliens, but all five together certainly can.
There's probably lots or even a majority of weird planets that are extremely inhospitable, complex life probably requires Earth's weirdness or close orbit of a gas giant, evolution probably struggles to make sapient animals, alien civilisations probably keep falling into the dysmemics trap due to unpreparedness, they probably destroy themselves sometimes, and they may not be as inclined as us to reach space.