But yeah, I also think about lost treasure and Indian Jones a lot with these stories.
People likely abandoned them for some unknown reason and moved to other locations, where they'd eventually get wiped out by the Spanish.
He was one of the survivors of the first European expedition through the Amazon and provides the only account we have of something resembling the pre-European Amazon. He talks about large cities with tall structures, roadworks, and very elaborate societies - nothing compared to our common notion of the Amazonian people.
His account was largely dismissed because it was never replicated, but those who believe it to be accurate theorize that this first Expedition unintentionally caused a population collapse by introducing European diseases. You can imagine that by the time other expeditions were organized, it was a completely different place and the jungle would have easily overgrown any abandoned cities.
Good starting point: https://en.wikipedia.org/wiki/Gaspar_de_Carvajal#The_Relació...
Replies like yours belong on Reddit, what an insufferable outlook on life and history.