Because he doesn't want to. And knows and admitted he would not and could not do it.
It's the [once again] media / fan base that comes up with this. I always laugh to myself. But don't say anything [well sometimes I do] when I hear people say that JJ calls down to change plays, who to play and all that.
I think one time he went down to the field, to question the status of a player after what seemd to be a very bad injury. Can't remmber who, was it Allen? turned out the player was ok to go back in. So they said JJ told them to put him back in.