The fundamental reason is that genetic diversity simply doesn't warrant it. You may think we're different based on appearance; but the genetic material says ot

The fundamental reason is that genetic diversity simply doesn't warrant it. You may think we're different based on appearance; but the genetic material says otherwise. I might be genetically much closer to someone of different skin color and gender than to my closest geographical/ethnic/cultural neighbor who looks just like me, in terms of DNA variation. Human beings altogether are not diverse at all genetically compared to most animal species. And if we were to use said differences to classify "races" among the human species, you can be absolutely sure skin color wouldn't correlate, nor would eye or hair color. For better candidates, look at the HLA system for an idea of more relevant classification method (this one defines immuno-regulation profiles among us, and has been shown to drive sexual attraction towards opposites, hypothetically because it favors enhanced, combined immunity in the offspring). But even taking such systems into account, human beings are really far from meeting any common threshold of differenciation worth being called "race" in the animal realm.