Why does God allow evil? You are asking for a reason for evil, but evil is by definition that which has no reason. It is the absence of what should be there. God cannot have a reason for it. It cannot be good that evil be. It cannot be that good relies in any necessary way on evil. That would mean that good isn’t entirely good and can’t be self-subsistent; it means that God, Who is Subsistent Goodness, doesn’t exist. If you think you have thought of a good reason for God to put sin in the world, repent and put such wicked thoughts aside. It is not better that Satan fell and Adam sinned.
What then, can theology do? It must be humble. It can only establish that a good God has no obligation to build a flawless world. There can be no reason why He didn’t do so. That uncreated perfect world with its uncreated sinless men would have a far better claim on existence than our world and we in it do, if it were true that anything could have a claim on existence. But it can’t, and this is the key point. We must not imagine that we exist independently of God like a child does from its parents and that we are in a position to confront God with obligations to us. It is a gratuitous act of His to make us exist at this moment at all. There may not be a reason for this act of creation to stop short of full bodily and spiritual integrity, but there’s no reason it can’t either. He can carry His creative act as far as He wants.
Of the “reasons for evil” I’ve read, only Zippy’s avoids justifying the unjust by appealing to the goodness of this world. There’s no claim that evil makes the world better than it would otherwise be, only that that other world wouldn’t be this one. It’s an appeal to thisness rather than essence. I’ve expressed reservations about it (though I’m not sure if I still take my own side in that argument), but it does have that virtue of reframing a discussion that had been set by asking a wrong and unanswerable question.