According to the WP article's section on epidemiology, possibly more than half of all people have a very weak form of myopia (0.5 to 1 diopters). The general amount of prevalence (as much as a third of population for significant myopia) is much bigger than could be explained solely by the proposed correlations (genetic or environmental).

To me this high prevalence and smooth distribution (in degree of myopia) suggests that it should just be treated as a weakness or a disease. We shouldn't act surprised that such exist. It doesn't even mean that it's not selected against, as CronoDAS suggested (it would only be true within the last 50-100 years). Just that the selection isn't strong enough and hasn't been going on long enough to eliminate myopia. (With 30-50% prevalence, it would take quite strong selection effects.)

Why are you surprised that such defects exist? The average human body has lots of various defects. Compare: "many humans are physically incapable of the exertions required by the life of a professional Roman-era soldier, and couldn't be trained for it no matter how much they tried."

Maybe we should be surprised that so few defects exist, or maybe we shouldn't be surprised at all - how can you tell?