If it turns out to be earlier than we thought then the climate may have been different. These sites could be the swan song of a civilization in decay, or a group that had an amicable diaspora and used these places for social purposes.

I imagine a culture that realized the land couldn’t support them in concentrations, picked a spot to meet every spring equinox to party and maybe make romantic matches and then dissolved back into the surrounding countryside.

Maybe this was their Burning Man.

Essentially someone had to figure out the civilized part of civilization and the density part of civilization. It’s a chicken and egg problem, and who is to say they did at the same time?

As GP mentioned, there were somewhat similar arrangements in North America not so long after this time period.