Skeleton busts his ass all week, coming in early and staying late (see Urbans blog) and working hard because even though all common sense says he will start, he knows that if he acts like the presumptive starter in practice all week, Whisenhunt will absolutely go to the other option (see Leenart, The Matt).
If the kid wants to be an NFL QB he shouldn't need the threat of not getting the starting job to do that.
Of course watching all the other slackers on the team could have caused him to think that wasn't necessary.