Um, how do you know he didn't?
Seriously. God could make many worlds. Perhaps one of them is without any evil. How would we know?
Maybe God has created ALL possible worthwhile worlds. This world we live in, then, would be one type of worthwhile world, but maybe there are other worlds that are very different to this world.
Anyway, an analogy: A good author can write more than one novel. And a good author can write a novel with evil characters, without the author themselves being evil for imagining such things (was Tolkien evil for imagining a world with Sauron in it?). God, the creative author of this world, is not evil despite the evil in this world. The evil in this world will not have the last say. It will be overcome. Just as great heroism cannot be displayed in a world of perfect safety, great love cannot be displayed without seeing what its opposite is - love is at its most loving, is shown to be most loving, when it loves the unlovely. How can we see the love of God, unless it is pitted against the greatest unloving things - unless we also see the unloving, and see it overcome by love?
Can God create a world with evil in it, and redeem it so that - when we see the total picture - we will agree: "Yes, it really was worthwhile creating this world"?
If a world could be created in which God overcame great evil and showed just how much love and wisdom and strength he has, should God go ahead and create it?