Why would you bother or care about the opportunities of some guy who you might never meet again, especially if you are trying to meet a deadline or had a bad day or whatever.
I've got fierce loyalty pretty high in my ethics, but there's a lot of pragmatic reasons too. On any one event, you could blow it off, but it's pretty obvious on the whole who goes to bat for their people and who doesn't. It's not something you make a calculation on - "Hmm, no one will know or care if I don't help this particular time." It's a way of life - take care of people that take care of you. Feels good inside, world sees it, recognizes it, and treats you well. A good thing, top to bottom.