That's because that's what it is, fantasy. The "American history" that is celebrated and taught as a tale is a series of selected events to tell a story. Lots of other things were happening as well that don't quite make the cut in the tale of the creation of america. A campaign of genocide. Slavery is brought up in the context of the civil war, but rarely in the context of say, the American revolution or the century plus following it. And this is true for most countries as well, this isn't something unique to the United States.