Yeah, it happens that way in relatively recent History. But today's hunter-gatherers societies are rather democratic. So too are villages of traditional subsistance farmers (e.g. Africa).

So, imho, in the past, such societies used to be democratic (primitive form). But some developed new tech and started using it to dominate the others.

Those that resist got killed off. And the rest surrendered (until it becomes too much, then, from time to time, they would rather uprise & die fighting than continue being oppressed...).

Those uprisings bring only incremental improvements though. As people are quickly satisfied by the little progress they make, and let future generations continue the job...