Because most of the population is considered intelligent? Hell, every generation thinks the previous one is less intelligent than the current one. As a species we are so full of hubris as to believe we are smarter and better than we actually are. That we think we are alway correct in our assumption. You believe we didn't build back then yet there is evidence to suggest we did, only for cataclysm events to wipe it from known history. Doesn't even take much for it to happen as our currently accepted history has shown.