Why do eyes go bad if humans are able to evolve?

Individual humans don't have the capacity to evolve; evolution is a feature of a species as a whole. With that said, I'm not sure what you're asking; do you mean why does eyesight fail? There are a number of conditions which cause eyesight to decline; macular degeneration, nerve damage, brain damage, lack of vitamin A, age, and even good old myopia, which is the medical term for nearsightedness.