Tribes constantly warred with each other, because they saw that as a good way to get resources.

Of course, during the middle ages it was feudalism, which was not centralized at all, it was stratified. Classes, classes, classes. Haves and have not-s. The haves of course arranged things in a way to keep what they have, so they come up with rights for themselves, which were very much not egalitarian ones.

And of course there's the real centralized one-party autocratic setup, where instead of noble houses openly competing with each other, people just play the subtle game of extort those who are under you, which makes inequality even worse.

This is a relatively recent phenomenon. To have oppression, there has to be enough of a surplus for a ruling class to extract. For many 10's of thousands of years, our ancestors didn't have the means to have a surplus.

Also, we have many examples of local societies which are egalitarian, but embedded within a larger, distant feudal hierarchy. In some of those cases, there can be very little violence directed locally, though in other cases, people can be so low in society, again, their only hope for advancement is violence.

Where there have been increases in the general standard of living, it has been through the adoption of new technology and the increase of trade.