To add to this, there are different rules for the "ineligible player downfield" penalty. In the NFL, only the players at the side of the formation, the "gunners", are allowed to run toward the punt returner as soon as the ball is snapped. All the rest of those along the line of scrimmage can't go more than a yard forward until the ball is kicked.
Under NCAA rules, the linemen can run toward the punt returner right after the snap. So many of the linemen take advantage of this and run forward after the snap. So if the college teams want to have anything resembling a return, they need to go back right away to help blocking. So the punter can take the 3 steps, run around and such as there's less pressure on him. In the pros, that time simply isn't there.