Why sins & death?

All sins are branches of a tree. What tree is this?
It is known as "Shirk", or "polytheism".
So, we who want to be pious and successful, want to uproot this tree & wipe away its remnants, its branches.
What removes the branches (sins)? Seeking forgiveness.
What removes the roots (shirk)? Pure monotheism (known as 'tawhid'); wherein religion is made sincerely for God Almighty alone.
This was the lesson given to Adam and Eve. Adam was intended to be placed on earth as successive authority prior to his creation. Laid out for humanity was an admonition against the Adversary and against the worst of sins & what leads to sinning - shirk - and they were bestowed Original Forgiveness (the mistake of eating from a tree was simply a mistake -- which they repented for, and were forgiven), an understanding of repentance & a clear path to guidance for guaranteed success in this life and in the eternal hereafter.

We die because life & death were created as a test for us -- our eternal abode is in the Hereafter, wherein we will be among those that either fill Paradise or fill the Hellfire.
We were meant for somewhere else, just as Adam and Eve were meant to learn about repentance and meant to receive guidance.
Our dying is not a purposeless happening, or consequence outside of our control -- but intended with great purpose, and each is held accountable according to their own deeds.