You are much more expensive, so companies are extra cautious hiring you. And it's just common sense that there is an order of magnitude fewer managers than there are individual contributors. And many (most?) companies have a bias to promoting internally.
I recall interviewing someone who had just a terrible technical interview, but honestly seemed like a good fit. In the interview it came out he had worked for and with two people who I knew fairly well (and respected and trusted.) Two phone calls later, and based on their experiences with him, he was on to the next round.
Okay, maybe that's not a fair argument on my part because you're actually using the positive feedback that you got from the mutual acquiantances and they could have given negative feedback, but it's still the case that some candidates don't get that opportunity at all.
In this case it was a benefit both to me and to the candidate.
Does knowing people in a field give someone an advantage