I think there's not much evidence of extra-terrestrial intelligence and what exists is rather incomplete, but it sure makes more sense to me than there was some massive slave economy that wasted all of that slave labor moving big rocks.

Remember all of those slaves had to eat. So every calorie expended pushing those rocks into place had to be raised by an even bigger team of farming slaves who also had to eat. The cost in calories to raise those rocks is staggering.

But you can go on dismissing this conclusion as flippant. It's not like anyone will ever really know because the security cameras weren't turned back then.