Interesting idea. But if God created lots of different worlds why did He go to so much trouble with ours? He sent his own son to die for us so we could be saved even after we screwed up time and time again. And He knows we will keep on sinning. Are the creatures inhabiting the other worlds just as bad? And if so why can`t God improve them (or us for that matter)? Or does He not want to?
It's not a matter of being bad. It's a matter of being created with free will (that is, the will to choose between good and evil).
A human can't have strength of character unless he can face the temptation of evil and overcome it. And even that strength of character seems to be a matter of choice (i.e., choosing to have Jesus and the Holy Spirit guide and strengthen us in our fight against temptation).
Being created to be perfect would make us little more than robots...and not very interesting robots at that.
Possibly Earth and Humans is just one experiment that God is running. Inhabitable planets are very distant from each other, and advanced civilizations are brief, possibly well under a hundred thousandths of an inhabitable planets existence. In combination these conditions provide excellent isolation between God’s experiments to prevent cross contamination.
A God that can create billions of stars and galaxies can easily attend to each civilization during its brief existence.
And it’s not clear whether humans were the objective of the earth experiment — maybe it was octupii or whales, and humans are an after effect that has to be cleaned up. He did wash away all but 8 that time.