Real answer: because imperfection is superior to perfection...or would be if perfection wasn't nonexistent and effectively the question is wrong. What is superior to the null set?
Imperfection is boundlessness. Perfection is bounded. Perfection is so bounded the boundary collapses to nothing.
But of course, imperfection is imperfect. Hence the sin. In a sense, you are granted the power to sin so you gain the capacity to stop sinning; in the long run it is well worth the cost. This is the capacity for boundlessness.
Put another way, is it infinite and eternal if it cannot sin? It is not: it has a boundary at sin. If it's not infinite, it cannot be eternal. Again, perfection is only possible outside existence. Timeless and placeless nothingness can be perfect, nothing else.
Your capacity to sin is what connects you to the infinite. Through the ability to sin, you touch the Dao. Would you sacrifice the Dao to buy a sinless nature?
While the idea is to not actually do it, of course, you cannot perfectly adhere to this plan. Perfection doesn't exist, and you wouldn't want it even if it did.
P.S. Timeless and placeless nothingness is a half-decent description of Khaos. Kosmos is imperfect; Khaos is perfection. At least, includes omnipresent perfection.