If you give people who didn't get a job the chance to anonymously trash the reputation of a recruiter, the veracity of the reviews are going to be garbage.
I think this is overly cynical - the vast majority of people, particularly those interviewing for high-skill tech jobs, understand that the recruiter is the messenger when it comes to the actual decision.
The problem is these review sites rely on complete honesty. Simply having "the vast majority of people" being honest isn't enough as it doesn't take many malicious actors to ruin an anonymous review system. You can ask basically any teacher for their thoughts on something like Rate My Professor.
Even a simple boolean like that is still subject to mostly getting results from upset people. People don’t tend to fill out surveys or reviews unless they’re either (1) upset or (2) there’s a reward (like “$1 off your next order” surveys)