That's because (I think) most black people go to public schools. Public schools do a HORRIBLE job of teaching history that predates much before 1964, to say nothing of the history of the Mediterranean, North African, and Middle Eastern region. Public school teachers go just far enough back to explain that our Founding Father's were a "bunch of slave owning racists". That's where it stops.
I would wager that if you asked public high school seniors where Persia or Alexandria would be located today, less than 5% could give the right answers. And, I imagine less than that could actually point out Egypt and Iran on maps.
If you were to tell public high schoolers about a tribal people who are indigenous to Africa, I'd wager almost all of them would presume those tribal people were "black".