Editing in a TL;DR: imagine you and your friends are thrown back in time to year 20 000 BC or thereabout. Imagine you find the nearest tribe of humans, and by magical means are able to understand and speak their language. Imagine you go to their chief and propose to form a confederacy, and ponder what would stop them from replying "ugh" and bashing your head in with a club. Compare with a closest analog to today, and where the difference comes from.

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> I'm genuinely convinced that prehistoric humans, being literally the same species as us, were just as capable as us in the ability to thoughtfully construct their societies.

I agree. We're basically the same people as we were before, hardware and firmware, +/- lactose intolerance and some extra mutations that, without modern medicine, would prohibit one from successfully reproducing. With that in mind...

> Like, why, when they bumped into each other, couldn't they have formed a confederation?

Because they most likely couldn't have even conceptualized this that long ago, much less make it work.

A "confederacy" isn't some built-in human feeling. It's advanced technology. Social technology, but technology nonetheless. In a way, it's merely a more advanced form of a bunch of elders getting together to deal with a problem affecting all of their tribes - but this is like saying passing around crude drawing on stones is basically a bit less advanced e-mail or international postal network. As an advanced social technology, a confederacy has a lot of prerequisites - including writing, deep specialization of labor (allowing for both rulers and thinkers to thrive), hierarchical governance, a set of traditions (religious or otherwise) that solidify the hierarchical governance structure and some early iteration of a justice system, literate ruling class, etc.; all of those are but a few nodes in the "tech tree" that leads to a confederacy, and more importantly, enables scaling the society up to the point we can even talk about a confederacy as we define the term today.

> I think instead of labeling them as children of nature or starving savages warring with everything in their vicinity, it makes most sense to see them as more or less similar to ourselves.

We still are children of nature. We're not starving because of all the advancement in science, technology and social technologies we've accumulated over the past couple millenia.

Consider that it is only recently - within the last 150 years - we finally stopped going to war over land and natural resources. Human nature didn't change in that time. What changed was that we've expanded to the point every place on Earth's surface has someone staking a claim to it, that the knowledge of these claims quickly becomes known to other groups; we then fought it out in 1914-1918 and then for the last time, in 1939-1945, then most countries accepted agreements to keep the borders as they are, and then we invented nuclear weapons and froze the borders via MAD.

The modern world is a beautiful but fragile place. If we let any of the supporting structures - whether social or technological or military - snap, the whole thing will collapse like a house of cards, and the few people that survive it will be back to prehistoric savagery. Not because they'd suddenly get dumber, but because they'd have lost all the social and technological structures that makes humanity what it is today, and they'd have to rebuild it from scratch, the hard way.