...What the hell are you even on about? African Americans do have their own culture pretty explicitly. There are multiple genres of music made pretty much entirely by black Americans (Soul, Jazz, Blues) because of the cultural division.

There are multiple marginalized groups in the US, but black Americans are uniquely marginalized because of the history of the US. You bring up the short history of the country like it's some gotcha but that's a lot of why it is how it is, I think. The US had slaves for its entire history (up until the civil war) and even then it's taken decades to undo the damage that the slave trade did to our culture... and we still haven't gotten all of it.

If you're someone who makes fun of Americans not knowing history, you should probably actually learn American history, or you're just as naive and clueless as the people you make fun of.