That is wacky - the Sumerians get about a half paragraph in the school books, then they move on.
Years ago I reached some of the same conclusions on my own, “great minds...” ha ha. It seems to me that some sort of cataclysm befalls humankind. It seems plenty of evidence that humankind is much older than popularly believed or suspected.
On the other hand it seems strange that North America doesn’t have any megalith structures or artifacts or anything whatsoever. Pretty big chunk of real estate. Even just a couple few hundred years ago it was virtually uninhabited. No disrespect intended to the Indian, just sayin’. That boggles my mind, to think of the centuries that went by - no matter how old one thinks the Sphinx or Great Pyramids are or any of those sites - 5,000 or 10,000 years, one big empty continent, for all practical purposes.
North AM had glaciers down past the present US-Canada border - winters must have really sucked over most of the US at during those millennia. Better to build in warmer climates as who is to say that the ice would not move even further south then. So the megalithic builders built from Mexico to Peru, but no further south as were more glaciers.