But why would God allow Satan to do such a thing and prosper?
It’s a part of a complex scene.
In classic Christian theological understanding, the problems began with the fall of mankind. Humanity as a whole, in some manner whose details go unexplained (I can only propose as theory that it was like some kind of universal ESP possible to unfallen souls) united in agreement to join Satan’s rebellion against God.
At this point, the only possibility for humanity not to be lost to everlasting damnation was salvation. And salvation is a complicated story. It entails learning how to rebel again, but this time to rebel against Satan, while uniting around God.
And so a nip-and-tuck battle ensues. We never came up with this world. This world resulted as a consequence of the clash between the wills of God, Satan, and mankind.
This is one of the big unanswered questions