Why didn't the biblical god create this incorruptable world in the first place?
This is the first place. It is still being created - or, more correctly, we are still being created.
The point was not God creating perfection -as he is already able to do that.
The point is teaching us to create perfection.
Perfection can exist without the EXISTENCE of imperfection, even if it cannot be understood without the context of imperfection -but CREATING perfection means that one must be as God -KNOWING good AND evil -knowing that which maintains a perfect creation -and the creation already made, and that which makes it imperfect.
God KNOWS both good and evil -but God is also able to DO only good.
Man initially KNEW only good -but did not fully understand or know good or evil.
Now man is experiencing both good and evil -so later the potential will no longer exist. There will be no question -no doubt -no temptation possible.
We will know God, because we will have proved him to be true, and his government and omnipotence necessary.
Man became as God -beginning to know good and evil -and will also become as God -able to do only good.
As God told man... "you must master it"
Otherwise, we would simply be programs -not true individuals -not truly creative.
As we are true individuals, we must choose to do what is good -and choose to obey God -rather than a constant struggle against the unwilling to maintain order.
For this to happen, we needed to prove that God is who and what he claimed -and that obedience to him produces good -so we were allowed to live otherwise as we chose.
Once we have matured it, none will harm any other -simply existing with each other will be a joy, and infinite individual creativity will be a source of infinite newness -built on a permanent foundation.
Isaiah 45:6-7
That they may know from the rising of the sun, and from the west, that there is none beside me. I am the LORD, and there is none else.
I form the light, and create darkness: I make peace, and create evil: I the LORD do all these things.