It's more so the implied insult that comes with making such a generalized statement. U could say, for example, WhiteBoy625 likes women who are short, have small breasts, and natural waist length golden blonde hair. BlackLady437 is tall, curvy, and has a short black afro. WhiteBoy will not like black lady. That is a fair and totally fine statement.
When u make a blanket statement such as white men don't like black women, u r presenting bw as 1 negative monolith, and by talking about who white dudes like, u r holding them in some sort of superior light where they r the ones who have to approve of the bw, and not the other way around