Flies have compound eyes which allow nearly omnidirectional vision. Evolving a new eye when you've already got hair in the way is probably hard, since it would get in the way of a simple light sensitive patch as a starting point, and even if a specimen did mutate one it would probably offer a very weak advantage compared in a species that already has fully developed binocular vision and the ability to turn its head.

Some species have developed additional eyes evolving independently from their original sets though