Theory: Atlantis happened, and it was terrifying. The survivors came away from that post-Atlantis experienced obsessed with making really strong safe houses, and also in experimenting with human suspended animation/resurrection. These pyramids were not to bury a Pharoh. They were to save a Pharoh and his family, if the NEXT cataclysm occurs. The "void" was a secret, even to the Pharoh, so that a bunch of other guys and families could hide in there too.