Because mostly even well spoken black persons have a distinctive voice. It just depends on who they were around, what they absorbed on TV. Even Asian-Americans have their own voice yet sound, "white". ie:James Chen, Ken Jeong, John Cho. Little nuances. Esp. If they came to the US/CAN, at a young age. Though, in other countries it doesn't happen because you need to do pronounce words to a T to understand what's spoken
