The people who built the pyramids may not have been gullible and stupid, but those who believe alien intervention was necessary to do it definitely are.
So, these aliens travelled through space and came to Earth to show people how to build pointy monuments? why didn't they do something more useful - the Ancient Egyptians, judging by their mortal remains, had appalling teeth, probably caused by eating bread made from stone-ground flour with a lot of stone dust still in it, which abraded their teeth. Many mummies wiould have been in constant pain from abscesses.
Why didn't the aliens introduce them to a less lethal means of grinding flour, and a system of dentistry?