Likely because you haven't defined what it is that needs to get done. People who are able to Just Do It have already been through the process enough times that they know the exact steps they need to take to reach their goal or they have put in the time to build a roadmap and use that to guide their decision making. If you don't have a roadmap, the path forward feels really daunting and your mind and ego get overwhelmed.

It sort of works like this....

Your current thinking: "I should exercise, I will go for a brisk walk every day." Then you get some free time but aren't prepared to step outside. Where are your shoes? Headphones? How long are you going to walk? Where are you going to walk? It all feels like a bunch of work to go through and figure out... "might as well just put it off and do it tomorrow." Without a plan, going for a walk isn't as simple as it originally sounded.

So make a plan. Break it down in to subtasks. First, define your brisk walk. "I am going to walk around my block twice". Then make a list of all the things you need to do to get that accomplished.

Put sneakers on
Put shorts & shirt on
Grab headphones and phone
Pick a playlist or podcast
Walk around the block once
Walk around the block again
Now, all you have to do is complete the simple subtasks.

So if you have all these great business ideas, choose one and then break its execution down into subtasks and focus on knocking those out instead of trying to do the whole thing at once. Apply this framework anytime you hit a mental roadblock, 9 times out of 10 there is always something that can be broken down into a series of actionable steps.

TL;DR: Learn how to organize your thoughts and align them with your goals.