There is a debate every year whether starters should play in the preseason or not. On one hand, it is benefecial to get some live reps against other teams, but there is always a risk of injury in football on the other hand.
The older veterans of the league almost never get any action in preseason and if they do, it is likely for only a series or two. Running backs will especially see lesser time in preseason as they take enough of a beating through out the year.
Younger players and rookies obviously see more action as well as they become acclimated to the speed of the NFL.