Well we did. We only in the past few years realized how the Romans made cement, which was a far more advanced building material than we had again until the industrial revolution. The Bronze Age collapse is an older example, but we’ve had stories of great civilizations along with the technology they control dying and leaving mysteries for those who came next for as long as we have a cultural record.
The leap to space travel level technology is a big one, but I could see it being a really cool sci-fi novel. I think that’s basically the only requirement for a conspiracy theory: does it sound cool.