What the Web Lacks: A Social Network for Finding a Mentor

Possibly because the connection usually works the other way. Usually, mentors find themselves a protege, proteges don't find themselves a mentor. Supply and demand works in favor of them, since there are many more people seeking knowledge and advice than people willing to take the time to mentor someone.

And usually, the connection happens at a subconscious level: you see someone who reminds you of yourself at a younger stage, or who you think shows a lot of potential but just needs a little guidance, or who you just like. It's hard to gauge that without a fair bit of time in an unstructured unrelated environment. That's why you find a mentor - it usually happens through serendipity.