The reason we should be paying for foster kids' college is because the state is their parent, so it's our responsibility. In a country that wasn't shit, regular people would be jealous of how kids who were wards of the state lived, and how well-raised they were. There's no clearer illustration of our values than the fact that children who, through no fault of their own, have become the responsibility of the state are treated like unwanted trash. The idea that a society like that could figure out how to ethically treat prisoners or immigrants is laughable.
I think nobody argues that it's a vile, morally repugnant thing to enslave another human being. But that was a long time ago, and all those slaves and the people who enslaved them are all dead.
The descendants of those slaves are now much wealthier and better off by pretty much any metric than their relatives who were not enslaved. How do you make an argument that those descendants are victims in need of reparations? No crime was committed against them directly, and they seem to have benefited from the crimes committed against their ancestors.
I must stress that this is not in any way excusing or justifying the wrongs that occurred. But how would you make an argument for reparations, given how things turned out?
Also the argument that descendants of slaves in America are better off than their counterparts in Africa is problematic because it assumes that the progress of African nations would have been the same without the devastating effects of the Atlantic slave trade, which significantly hindered their development. Furthermore, it risks minimizing the experience of ongoing racial discrimination faced by Black Americans.
The idea of reparations isn't necessarily about compensating individuals for specific harm done to them, but about a society taking responsibility for historic wrongs and making a concerted effort to rectify those systemic inequalities. Reparations could take many forms, including investment in education, healthcare, housing, and economic opportunities for communities disproportionately affected by racial discrimination.
Slavery in Africa was widespread before the Atlantic slave as well as after the Atlantic slave trade. There's some apologism (interestingly enough, quite similar to Southern U.S. slavery apologism) claiming that it wasn't that bad, but if you look at the actual accounts it could be extremely brutal. Like with the U.S. there was certainly a degree of different experiences, but like in the U.S., that doesn't justify the practice.
In the end it was actually European powers that ended most slavery in Africa, often with a great deal of local opposition ("The End of Slavery in Africa" is a decent starting place if you want to see how it happened in each individual area).
Ethiopia is an interesting example - it wasn't colonized[1], and so slavery there persisted long after it ended in most of the continent. The League of Nations kept pressuring the country to end the practice, but it kept dragging it's feet. It only ended when Italy invaded in the run-up to WWII (it's also interesting as a non-colonized control country when it comes to colonization).
[1] It was conquered by fascist powers for some years, the same as most of Europe.