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.