One has to wonder what our "hate running water" thing is - ie something we feel compelled to do which seems perfectly sensible to us but is really just an evolutionary gimmick that some other species would think is weird.
As far as I'm aware, cats don't care at all what colour fur other cats have.
Encountering someone from far enough away that they would be part of a distinct genetic or cultural group was basically unheard of. That's not to say it didn't happen at all, there would be migrations and smaller groups would potentially travel long distances out of necessity, but it simply was not a part of normal life. The idea that a fear of different looking people provided any meaningful benefit in the past simply does not pass the sniff test.
Further, wars were almost always between regional neighbors then as well.