Because black people do have different hair colors, nose shapes, skin complexions, different hair texture, and different eye shapes. I mean for the most part every time I have seen a white person I could I knew that they were white, I never mistaken them for another race. On the other hand I have met black people Who I though were middle eastern or hispanic or I thought they were mixed with white, hispanic, or asian, but they were not they were just black. I happen to be one of those black people who people think I am mixed with something but both of my parents are black my grandparents are black too. So that's probably why they say blacks are a diverse race.
Although white people come with different hair and eye colors and nose shapes, for the most part you can tell when white people are white.