>>8999292ooh ooh i can sorta answer this one
the first thing is that evolution is a tinkerer, not an inventor, it only slightly modifies things over time that are selected for. so the simplest answer is humans have two eyes because old world apes had two eyes, because the first mammals had two eyes, because the amphibious precursors had two eyes, because the first fish in the cambrian explosion or w/e had two eyes
so why would a fish have two eyes? well, if we look at fish today, their eyes are on either sides of their head, allowing them greater field of vision than one eye. why didn't they have more? because that would require more coordination and vertebrates have a lot of symmetry which makes having an even number easier to develop. it would require more nerves and more brain capability to process more complex imagery from more than two eyes, and two got the job done well enough that there was more cost than benefit to having more (insects and the like with more offset it by having less capability in their eyes - a fly has segmented eyes and a spider has multiple, but they are not as complex eyes as ours and are mostly for light sensitivity and close proximity viewing instead of depth and multifunction)
so basically it's that humans wouldn't gain much from having a third eye: we have depth perception and a good field of view for how our body is oriented. having any more forward facing would be redundant and pointless, and any side or rear facing would require some serious differences in the brain's image processing. so any changes to our current setup seem suboptimal - there's a word in evolution for being developed in a way that any small changes in either direction would be selected against, i forget it, but it's similar to the same idea as a local minimum in mathematics
