When you have 11,000+ employees, it's hard to have an objective team investigate every single review. Plus, how can this objective team, with little to no context of daily interactions, assess whether or not feedback is "legitimate" or dripping in disdain?

Yes, that's called 360 feedback. It suffers from some intrinsic drawbacks, though.

First, nobody has an incentive to provide negative feedback except for serious cases (i.e. you cannot work with this person) or for political reasons. It's a prisoners dilemma with indefinite iterations, so each player's incentive is to provide positive feedback.

Second, in my experience, it's only reviewed by one person rather than by a management team, which opens it up to an array of biases on the manager's part. If a manager thinks that someone is a great team-player, and a reviewer says that that person is a petty tyrant, they're likely to doubt the reviewer's motives rather than updating their beliefs.

Yes, and there are some problems with that system as outlined by others — I've also seen instances where the system was really only half-implemented:

Peers supply the feedback, but one manager has the time/resources to review it... so basically they can interpret it however they decide.