Offensive linemen go into college knowing they likely won’t see the playing field for at least one season. It takes a long time for them to get acclimated to the college level, and to establish the proper strength and conditioning needed to play the position. Sometimes you get guys like Cesar Ruiz or Zak Zinter who are too good to keep on the bench, but for most offensive linemen, they sit a year or two and then start to see significant snaps on the field.
But sometimes, guys get up to speed on their strength and conditioning and just get passed by other players time and time again.