it’s a good question. but having good vision to the extent that we need it today (to read whiteboards, computers, books, etc.) wasn’t necessary for the majority of human evolution. sure, you may be a bit less capable at seeing that buffalo off in the distance, but it needn’t be sharp for you to see it, and as long as whomever you’re with is seeing it, you can still assist with the kill. maybe people with truly horrible vision were selected out, but in general, i don’t see why you couldn’t thrive and reproduce in primitive times with, for instance, typical near-sightedness.