Because they're idiots
If people saw my brother and they'd probably say he ain't black (skin tone like drake) but he a full negro that's for sure
People find it hard to differentiate between light skin and mixed race
Also tbh some not all Somali but a lot don't consider themselves black, those fools think they are closer to Arab than African lol I consider them black however
Black is your skin tone, I mean are people, suggesting mo farah ain't black cuz he has thin lips( non existent upper lip) and then nose like a white man but not your stereotypical wide nose big lips?
Also a lot of mixed people consider themselves as black people, some do look fully black despite the mixture, I.e Collin Jackson Barack Obama, even white folk in America consider him black the only people who don't are the same people who categorise them with black people if they have a negative vibe to them or a criminal.
People who say your not black because of your facial features are idiots, soon they'll tell you, people like Kerry Washington ain't either, goats