There is no satisfactory answer in the texts, but here's what I think...
A broken bone is something unatural. It's an accident, it happens to most people at some point in their lives, but it is not something that happens as a natural course of events in all young people's lives.
Nearsightedness is simply the body doing what the body does. It's in someone's genetic code that the cells in their eyes don't work quite as well as they should. Altering this magically is probably very unsafe and would only be temporary once the old, magically fixed ocular cells die to be replaced by new ones.
Case in point is Voldemort. He looks drastically different than he did as Tom Riddle, the result of his "magical experiments." His tamperings with things that ought not be tampered with lead to his highly unstable soul and eventual death. There may very well be a spell or potion that fixes vision, but most wizards don't seem to mind glasses, so it's more trouble than it's worth.