To be fair, I always thought tieflings, aasimar and genasi were human subtypes, and that the underlying magical assumptions of the D&D cosmology suggests that humans are all racially identical for stat purposes.....except when something magical changes them, often by birthright.
To look at this from a more metacultural perspective, it's pretty easy for us to all agree that one reason you don't start assigning statistics to human racial groups is because A: it's a creepy judgement and how do you quantify this, let alone justify it, from a mechanical or scientific perspective when our own human experience already tells us that the real world does not work the way D&D mechanics do, B: it's uncomfortably close to the false science of past eras and fringe groups today who want to make those judgments; and C: even if this were culturally okay for some reason, actual science indicates that there are not, in fact, racial differences that can quantifiably be identified, let alone applied using D&D mechanics in a narratively coherent fashion.
Or put another way: if I were Inuit and you told me I get cold resistance, I'd laugh at you. (EDIT: But per othr post, if you told me I was of the Winter Ice Men and my goddess had gifted us with cold resistance, I'd be like....cool, sign me up)
Anyway: D&D as I said already does this, and it does this by adding magical elements to human types which makes them more than human, and also magical. Once magical, the rules suddenly make more sense and anything goes, pretty much.