Or perhaps it's the market-based economy, supply and demand - a feedback loop that, by its very nature, optimizes away any average surplus a society accumulates, in the same way a DC filter will remove energy from a signal until it averages to zero.
I'm increasingly convinced this is a direct consequence, a flip side of our economic system - i.e. the very fact that people can freely compete means prices will go down to near production costs, as competition minimizes everyone's margins - but at a larger scale, it means the market will grow and change to consume any surplus, which, since nothing here is mathematically perfectly uniform, necessitates the existence of poverty and affluence - income inequality that can have arbitrary amplitude, but averages to zero.
> There are plenty of studies of Indigenous peoples whose societies are much more equitable than ours
Please name three. As far as I've read, all the various indigenous tribes past and present tend to have the same problems as every other group of people, including modern civilization; they're just earlier on the problem complexity/sophistication ladder. That, and they tend to have pretty horrible alternatives to what we've learned to solve via taxation and the rule of law.