Imagine you're a recruiter at Facebook. You reach out to thousands of candidates, convince hundreds to come in for interviews, and almost all of them fail because the interviews are hard and test skills that need to be specifically trained. If you convince a Googler to come in for an interview, they probably have a nearly 50% chance of passing since they've already done it at least once.
The recruiters are not exactly incentivized to bring in people who will be great hires. They are incentivized to bring in people who can pass the interview.