Because the NCAA can't sell jerseys specifically for certain players, only for the numbers. That's what they do, right?
For nearly all of its history, Penn State football has played with no names on the backs of its jerseys. One major advantage of that is for the fans. When their favorite #24 moves on from Penn State, they know that there will always be another #24, who could end up even better than the previous one.
There are plenty of Penn State jerseys with deep and impressive roots.
#22- John Cappelletti, Evan Royster, Akeel Lynch
#31- Paul Posluszny and Shane Conlan
#14- Todd Blackledge, Wally Richardson, He Who Shall Not Be Named, Christian Hackenberg
#8- Terry Smith, Rodney Kinlaw, Allen Robinson
I could go on, but I think you get it.