We're not saying it was aliens... but it was aliens.
Want proof that the aliens built the Giza pyramids? Simple. Each of those hewn stones weighed over two tons. There were no cranes back in those days. Are you telling me they just had some Egyptian Paul Bunyan carrying them? Possibly. But seems unlikely.
Also, each of the pyramids lines up precisely with the stars of Orion's belt, or some dumb crap like that.
Furthermore, any feat of ancient human engineering that we don't understand we can pretty much chalk up to aliens. Cuz why not?