Especially when it comes to big men, three star guys aren't often ready to play immediately.
Freshmen make mistakes. Guys with more raw ability recover from mistakes, blocking a shot from behind after being beaten to the basket because of poor defensibe footwork for example; and some coaches live with their mistakes because their raw ability allows them to be special at other times.
Three star recruits have to remove the mistakes from their games before they're ready to play. They spend a year or two improving positioning, perfecting footwork on both ends of the floor, and understanding team offense and defense. Then they are ready. Think Jack Cooley, Ty Nash, and Zach Auguste (Zach was a mix of 3 and 4 star ratings).