You capitalize "Black" for the same reason you capitalize "Spanish" or "Polish". It's a distinctive ethnicity and culture; it isn't simply a broad description of people sharing a skin color. You can read John McWhorter --- no friend to wokeism --- talking about how much better capital-B Black is as a description than any of the other weird terms we've come up with in the last 20 years.

Nobody anywhere on this thread is freaking out when people fail to capitalize "black". The people who do capitalize it are simply making it unambiguous that they are referring to American Black culture.