To be honest, the plausibility of the ancient aliens theory is about equal to that of religion... Both are the result of human imagination, tendency to look for patterns everywhere, even where there are none, and projecting our biases onto them. It's pseudoscience and baseless speculation - cool idea for speculative science fiction, though. Personally, I hate hearing about how aliens built the Pyramids, taught us cool things, etc. mostly because it suggests that "primitive ancient humans" weren't capable of actually accomplishing things on their own. Yes, they weren't as technologically advanced as we are, but they were not stupid, they had their own, efficient ways of doing things, and they most definitely knew what they were doing. It's not a mystery, we know how those grand engineering structures were built, and suggesting that it was due to some extraterrestrial intervention is taking the credit away from the amazing cultures who actually did it, while also downplaying tens of thousands of years of human ingenuity...