All it takes, though, is for one unscrupulous or lazy individual to join, and then they vouch for a few people they don’t really know, and then you get a bot vouched for, and then a few more, and the first few bots vouch for other bots, and soon they’re all vouching for each other and you’re back to where you started.

Basically, it’s a system that works at a scale where individuals can hold one another accountable. But not really beyond the Dunbar number, and certainly not at internet scale.