You can believe this only because you’ve been culturally impregnated by the concept of the existence of races (and more specifically of the three “races” you mention in a latter post).
It’s not “liberal dogma running against common sense”. In fact, as I wrote in a different thread about racism in Roman times, it’s the belief in the existence of races that is very counter-intuitive and requires quite a lot of cultural baggage.
Not only the concept of races doesn’t make sense from a scientific point of view, for reasons already or soon to be mentioned, but it doesn’t make sense either from a common sense point of view. One could give tons of examples, but for instance : in eastern Africa, where does the “white race” end and the “black race” begin? To what race (caucasian, negroid, mongoloid, according to you) do black-skinned southern Indians belong to? What is the race of the current American president, and why? Do Pygmies and Dinkas belong to the same race and why?