Any opinion on why AA sponsor should be someone you do not know.

Any opinion on why AA sponsor should be someone you do not know. I mean do you think it would be helpful or not to have 2 close friends or 2 members of a family work as a "pair" to help one another as AA sponsors? I have never heard of a close friend or family member being someone's AA sponsors. My nephew attend AA meetings and he doesn't know his sponsor.