Visible history is generally written by the beaurocratic/violent society, not their egalitarian neighbour. If you think further back in history, I'm sure you'll recall "the great periods" as we call them, marred with obvious artifacts from oppressive societies.
Their egalitarian neighbours where only just starting to discover. In part because their traces are less obvious, and in part because we've had a mental confirmation bias against finding them. I refer back to The Dawn of Everything if you want more details!
This is going to sound sarcastic or something, but I'm actually curious -- do they? I would tend to assume that they appear to come in pairs, but this is largely because we happened to find history books written by the self-professed egalitarian society, and not their rivals, to whom they allocate every vice.