They didn't even properly ended slavery until recently what with indentured service, Jim Crow laws, biased judiciary and all that.
I worked in the bible belt of the USA in the nineties. I was deeply shocked at the overt racism. I was even more shocked at how downtrodden and separate the black community was. Very striking how few mixed race families existed. I did not once get chatted up by a black man when out and about but white men were quite happy to approach me. The whole society seems set up to entrench the idea that race is real and exists as fixed boxes like black and white are clearly separate categories. Never occurs to them that it is weird to not have loads of mixed race families like you'd get in European countries. IMO, their daily experience is of quite separate communities and just goes to reinforce the stereotypes. Failure to travel makes it worse.
It is always fun to bring my US colleagues to London, Copenhagen, Paris or Birmingham and see how much makes them give a double take.
I worked in France in the eighties. There was vile abuse towards "Les Arabs" that surprised me.
In Ulster the separation of catholic and protestant was also an eye opener.
Asia's splits too were surprising to witness.
I have come to the conclusion that all peoples are inclined to be vile to those who are "other". Every place has some group living there who are somehow kept separate, whether by historical accident, separatist leader pressure (e.g. religious leaders and wannabe lords), or accident of geography. Whatever the split, there is an ism. In the US, they made the split on race and it is still strongly entrenched. God knows how they can fix it.
One of my mixed race cousins thought of working where I did in the US. I told him to avoid like the plague because he looked too black and would have a shit time. And possibly even get murdered because he, a Brummy, would not give a second thought to chatting up a white lass.