First, a nitpick: I didn’t say God “creates and sustains” evil, only that God allows evil. Which may or may not be an important distinction; God did at least create the possibility for evil to exist.
You do have a good point. But I can think of a couple of possible ways of answering it:
Number One: There may be some logical impossibility inviolved in “just creating the end.” For instance, can heroism exist without something for it to struggle against? Can free will, or choosing the good, be possible if there is not also the possibility of choosing evil?
Number Two (although this might just be a variation on Number One): God may have deemed it more “artistically satisfying” to let the good win victory over evil. Simply willing it to exist from the beginning would be like an author who took his characters straight to the happy ending without letting them go through trials and tribulations to get there. We appreciate good things more if we had to work hard to attain them, and we appreciate good times more if we’ve been through hard times on the way to them.