Did aliens build the Coliseum and the Parthenon?

I know they didn't; Bigfoot did. My real question is why do some people automatically assume Aliens built ancient monuments like the Pyramids. Why do they think ancient peoples (usually non-Western ones) were too stupid to figure it out? Don't they realize how insulting and condescending it sounds towards the ancient peoples and their modern descendents? Doesn't it say more about our lack of ingenuity and creativity that we can't figure out how pre-modern people did it?