Because your statements aren't based in reality.
On paper you'd be correct. But this is America. Race is still largely viewed as a binary concept. Light folks were still included in the "black experience" whether you acknowledge that or not. Membership to extremist groups hasn't risen because the President is perceived as white. DuBois didn't have problems at Harvard because he was viewed as white.
Socially, light and mixed folks are viewed as black. If you're talking about genetic purity or whatever, good luck finding a pure ANYTHING in America.
Now the problem with calling light and mixed folks out is it invalidates their accomplishments and commitment to the whole.
That and it's kinda presumptuous to tell someone that they are or aren't black because of your inflexibility.
The issue is mixed people are being used as the main face of what is black.
When they are just a small minority