To put it bluntly

In short, the κύριος πάντων [lord of all] must be divine (or at the very least uniquely connected to the divine). Social contract advocates since Rousseau have naturally tried to dispense with this necessity—as has the modern world in general—or at least to restate it in terms of the power of “the People.” But the ancients knew better. Ultimate sovereignty entails divinity. The Roman emperor was a god.—World Upside Down, page 111