Agriculture is the culprit. As hunter-gatherers, there wasn't enough food to create winter stores, and we had to migrate towards the equator during winter months and back during the hot summer. Then we developed growing enough that we could homestead and stay in place with winter stores (and get cabin fever).

Then raiders figured out they could raid homesteads, and some raiders figured out they could just tax the homesteads in a protection racket, though it meant protecting them from other raiders. And bam! Feudalism!

(Later on we'd realize, with generations and generations, that one Joffrey or John of England or Caligula can bring to ruin the legacy of ten fair and just kings prior to them, and we didn't really get fair and just all that often. Now we're still trying to figure out how to prevent aristocrats, plutocrats and autocrats from consolidating power away from the people. It's a sociological development we don't have yet.)