Some people think it's down to racism, that the people saying it couldn't be the Egyptians (etc.) can't believe that a people they viewed as hopelessly backwards could build something that impressive.Builders at the Gt. pyramid left their graffitti in secret spots to tell of their labour, not to mention the records kept by the foremen who ran the project. Why take away one of the great engineering feats of the ancient world from the people who worked so hard to achieve it? When the knowledge was lost they'd claim they were built by giants...now it's aliens, by people who can't be bothered to unchain their brain from a banal beleif that "the truth is out there".
Notice that no one seems to think the Romans or Greeks were assisted by Aliens.
