An extremely good point! You almost never see ethnicities of demi-humans or aliens done naturally in this type of shit. Ethnic variants come about via long-term evolution in different climates. There's a reason people can look as different as they do, and it's not culture-based. But then you put someone like that in a setting where they are the odd one out, for dIvErSiTy. It makes no sense in medieval fantasy settings. Star Trek though, everything is already being brought together by the Federation, and the various races are all space-faring, so cultural isolation can only be a choice rather than nature taking its course.