Just peruse the list of great works that Ancient Aliens proponents claim could not have been built by the people who built them. Do they make that claim about the Parthenon? No. Other than Stonehenge, it's all stuff built outside of Western Civilization.

It could be a series of coincidences, or it could be old strains of racist anthropology, briefly suppressed by Boasian Cultural Anthropology, finding a new conspiratorial outlet.