It's the players choice to do which drills they want, and most players choose to do all the drills or at least the majority of the drills. And this includes drills that have little or no bearing on their position, why else would a kicker be running the 40 or doing the bench press?
Believe me if it were up to scouts which drills which players participated in then you wouldn't see OL's running the 40.
The media has hyped up the 40 and the bench press because they are flashy stats, this has caused both uninformed fans and players(most of whom are told by agents and trainers that doing certain drills will not help them in the eyes of NFL scouts but may raise their profile in the media which may help them in different ways) to put some stock in it.