At least speaking for myself, my compulsory education only really focused on slavery as an American institution and didn't really address it in a broader context. I obviously picked up a little here and there, particularly since I took Latin all through high school and learned a little bit about Roman culture, but it was only recently I learned how widespread slavery really was.

As an example, it was news to me that slavery was still practiced in parts of the Middle East until the 60s, and probably still exists in a less obvious manner today. I was really horrified to learn that tons of slaves left Africa via the Eastern trade routes, and the brutality of those owners is why there's little to no visible evidence of slavery in those parts of the world. Evidence meaning descendants of said slaves.