The pyramids at Giza used to be smooth on the outside so people took pieces of them and built something else. I think they’re in a category with places like Angkor Wat. The sites’ importance decreased (religions changed, trade shifted, natural disasters, etc.) and it was easy for nature to cover them in sand or jungle and, poof, out of sight, out of mind.

It is very likely that they weren’t in fact totally forgotten. There probably was local knowledge about them that led white men with too much time and money, thinking themselves superior and as preservers of culture, to “discover” them. Tourism was for the elites and there wasn’t any money yet in preserving these old sites.