Part time AWD

It seems that everything that has part time 4WD, can be switched from AWD to FWD...so why not RWD? I always thought Front Wheel Drive was for family cars, because that way its lighter and they can free up space in the rear of the car by having no drivetrain there...but since with AWD there is already running gear extending to the back, why not RWD? Wouldn't it stop things like understeer(although I don't really understand it) and that?