In hi-school they get by with pretty much only using their upper body. In college, they will probably need their legs over their upper body more than 50% of the time.
Centers are rarely ever ranked high out of highschool. If you are a 5 star center you can probably learn to play tackle because you must be athletic enough to get that ranking. So why not slide to where the money is?
I agree with what your saying in a way and data does back up high blue chip ratios leading to wins but you cant seriously watch Tarquin's(4*) tape and say a guy like Darius Washington(3*) is not better. Sometimes you gotta use your own eyes.