I think it's trust. If you worked with someone and you know they are awesome and you are running a team you want to hire people who are known entities that you know will hit their deliverables, do a great job and won't be weird or cause friction with cross functional teams. Also of course there's always politics to navigate. I had a candidate I loved for my team who made it through 8 rounds and panel interview with strong hiring recommendations but we got overruled by our skip level who wanted to hire her friend.

The interesting thing is people recommend "networking" and people think it's like... meeting people at industry events or reaching out to linkedin connections but that's not really how I see networking working in real life. The networking that actually gets people jobs is networks from people you've already worked with before, or your professor is on the board of the company or like.. your parents know the exec team really well etc. So new grads truly are disadvantaged.