Because it essentially requires everything known about the recent past to be wrong. Human societies and civilizations are incredibly complex and don't just pop out of nowhere and then disappear without a trace. We have an overwhelming amount of evidence on the human development from genetics to radio-carbon dating, to deciphered languages to the spread of technological innovations.

It is as nonsensical as claiming that the latest Ryzen is really from the 1990s. Of course, this guys 'theory' is not based on actual evidence; so there is no reason to even take it seriously.