The job of a running back may seem very simple; absorb the handoff in your gut from the QB, do not drop it, then avoid the tacklers to go as far as possible. Rinse and repeat. From a purely positional silo standpoint, you can make a great living doing just that.
However, in today's game of complex offenses there is so much more involved. A good running back has to be able to hold onto the ball and avoid defenders to even sniff the field, and they also need to do many other things to STAY on the field and help make a team successful.