Everyone works closely together, often bouncing around jobs and having friends at other restaurants. Having this site totally anonymous gets in the way, but reputational damage remains possible and would complicate extortion efforts or legal liability the site could face.
Service staff who work up front with customers gossip like crazy and tend to be aggressively managed.
Kitchen staff are often salty characters who bring their own knives carried in a fabric or leather roll. Giving them guff is a bad idea.
Omertà is a mafia-like code of silence. A bad review that makes things harder for those left behind can draw reprisals, and some managers take reviews psychotically over-seriously.
I’d be surprised, given how restaurant folks are often tightly networked. Front of house politicking and back of house knife-rolls are both good for omertà.