I'm pretty sure that there is no method for levelings jobs up within the game, without essentially adding them as separate classes (which would be a giant pain). So much easier for the Devs to just limit them to the rank you are at.

Though, I would be OVERJOYED if you started off your job at whatever rank - but with your best abilities all locked, and you had to quest to learn them or unlock them. That requires no ranking up but accomplishes much of the same purpose.