It's the authenticity, but even more than that it's the saturation of inauthenticity. Even if there's oodles of authentic content, if there's enough inauthentic content to drown it out, you enter a vicious cycle where plummeting interactions and new authentic content both deed each other.

I have a hypothesis that network effects kick in for social interaction before they do for monetisation, which is why the advertisers/influencers/propagandists/scammers(/trolls, though this is different) are in a constant state of hunting down and infesting whatever platform good-faith users have most recently fled them too. Part of it is likely that smaller communities are more robust and have an easier time identifying and repelling smaller-scale incursions, but I suspect a big part is that smaller communities simply aren't worth the investment of larger incursions, especially since they'll more easily be ruined before any real payout.

Anyway, I agree with you that "quality" (as in effort and craft) is lower on the list of factors than authenticity, which makes complete sense. There was a time when a well-crafted ad was worthy of note, but ads have been so sneaky and pervasive that I think many people are desperate to have a spontaneous interaction or experience that's not trying to sell them anything.