After Adam did his aveira he was expelled from Gan Eden. The Chasam Sofer asks if Adam was supposed to be in Gan Eden, why did Hashem build the entire world outside Gan Eden in the first place?
He answers that in the original Bri’ah, Hashem created each thing in its full glory. The cows were created as adults and the trees and vegetation were in full bloom. Similarly, man was created in his height of perfection. Therefore he was worthy to be in Gan Eden. Even if Adam had not sinned, the next generation would be born from scratch as imperfect children, just like the next generation of trees grew from the ground up.
It is a person’s job to do mitzvos and perfect himself and only then be admitted into Gan Eden. It is for us that Hashem created the world. Unfortunately after the Chait, Adam as well became imperfect, and was sent out to the world to do tshuvah and perfect himself for re-entry to Gan Eden.
Another point to this answer is that Hashem gave us free will so that it was up to us to earn our right in Gan Eden. Had Hashem just placed us there when we were born we would have no free will!!
