It’s what they’ve been told. And of course the United States was the only nation to use slaves. /s
It was a despicable practice. It has always been around. Today it still exists, but not in the U.S. None the less, you never hear slavery trashed where it still exists.
Instead the Left pumps out lies, and the misinformed buy them lock stock and barrel.
. . . and that's because it only exists outside Western Civilization and is practiced exclusively by non-white peoples.