Because these buildings were most likely not built but discovered/found by the American colonists and the natives. They simply dug them out of the mud. Sounds insane. As I said, deep rabbit hole. Look into it if you're curious about it.

Massively scaled buildings allegedly built "temporarily" using wood and plaster for the world's fares around 1890-1920.

Scepticism about canon history is pretty widespread. Lots of Germans (not because of WW2), Russians and Eastern Europeans have cast doubt on the middles having existed for example and they're all well respected in high positions, like Analoti Fomenko for example. So no crackpots.