I think the thing too is that being a coach is really hard work; 16+ hour days, lots of travel/time away from family, working with other stressed out assholes, etc. Most successful QBs have hundreds of millions of dollars in the bank; they have no reason to subject themselves to that type of punishing lifestyle. EDIT: yeah, pretty much completely what G said.
Probably why a lot of backups/ham-and-eggers end up on the sidelines and the successful guys end up as media personalities; the backups are only worth a few mil and can't coast for the rest of their lives.