Why? Because the are.
Why can Jaqen change his face at will? Because he can.
I don't need to know his genetics or study his DNA to roll with that, it is what it is...fiction. You're the one that needs racial homogeneity studies to accept Black folks ruling a Britain-like area. I find humor in that.
But the point of Westeros is that it's an insulated, insular society. They literally built a 700 foot high wall across their only border. Having it be racially diverse changes the whole dynamic of the place, does it not?
For instance, say there was a random house in The Reach which consisted of Black folks. Okay, so are these people accepted in wider Westerosi society? If so, that makes Westeros seem kind of tolerant of people being "different", right? So then the whole aspect of Dany's potential invasion with an army of "others" doesn't carry the same meaning.
Or what if they're not accepted by the rest of Westeros? Then how did they survive without being persecuted to extinction?
I can easily accept Black folks ruling a Britain-like area if that's what the story is. Surely you can accept a racially homogeneous bunch of white folks ruling a Britain-like area if that's the story, which it is in this case?