I'm far from an expert on this topic, but I believe the reasoning is that (at least in the USA), African-American ancestral links don't necessarily go back far

I'm far from an expert on this topic, but I believe the reasoning is that (at least in the USA), African-American ancestral links don't necessarily go back far enough to identify an ethnic group because of slavery. In the absence of the ability to say e.g. "I'm from the Yoruba people", the primary group identification just becomes Black. I agree that it seems kind of reductive from the outside, but I'm mostly inclined to let people decide their own names.