Every organization has its unwritten rules. They're not intended to be devious or debilitating. But they can be.
A mentor is not the only way to learn the unwritten rules that determine success in your organization---trial and error works fairly well (takes a little longer and can be a bit painful). A good mentor can also help you learn the principles and approaches of effective leadership.
Why aren't there more mentoring relationships? Maybe there will be after you read the attached article, "Find (and Be) a Mentor and Protege"!