Because blacks in this country want special undeserved privileges related to race. In my bloodline, I actually have some Latin American and North African mixed in on my fathers side a few generations back. However, I would never claim either on any application nor would I try to pass off either as being part of my identity so people would think I'm "diverse".
Unfortunately, being 1/1000th black qualifies you as black in the eyes of the black community. It's identity politics at its lowest common denominator.