I do not believe the bible addresses this but it’s a valid question and I do not know how to answer it.
Why did God wait 4,000 years to send Jesus and another 2,000 + before bringing things to a close (the close obviously having not happened yet)? Why not have provided the sacrifice up front and wipe out the following 6,000 + years of suffering?
Yes God, why didn’t you make it easier on all of us? We know you don’t do things arbitrarily and we know there’s a lesson in here for all of us, but it sure would have saved us a lot of inconvenience. What’s that, God? Yes we know you’re the Potter and we’re just the clay but even so.
Seriously, aside from the things we’ve learned about the nature of sin, the futility of works-based religion, the incomparable riches of God’s grace, His nature and character, the unblemished accuracy of prophecy, the fact that even when humans know the outcome of their actions in advance, we still won’t obey, and a host of other things, it remains that He’s God and we’re not.
Even in the Millennium, with the Lord personally ruling Earth, all unbelievers banished, and Satan bound, there’s still enough sin in natural man to rebel at the first chance he gets. So I guess we still haven’t learned.