There's a part in this upcoming creepypasta I'm working on where the main character is being told a story about this human scullery maid, who was working for this rich elfin family, and happens to be incredibly ugly, to the point of Innesmouth Frog Person--horrifying, so much so that this particular family hid her ugliness under a spell of glamour. So the main character's wondering--"Well, why not just use magical plastic surgery on her and fix up her deformities then, instead of just casting an illusionary mask spell?"

I need a logical reason of why a seemingly easy magical fix isn't used to alter a physical appearance.