sometimes third round picks come in and start immediately (not very often, especially on the O-line). sometimes they need time to "develop." spend some time in the "weight room." get some "coaching." recover from injuries. learn their trade. then, in their second or third season, when an opportunity to play presents itself, they are given a chance to show what they can do. if they can't play, they get cut.
ideally, every player drafted by the general manager of the __________________ (insert your favorite team's name here) would fall into the former category, come into camp like a tsunami, embarass the veterans ahead of them with their stellar play, seize the starting job and make the next ten pro bowls, but it just doesn't work that way.