If the box is clear before every snap, do you really care how many are in there during the dead ball?
It's not a big deal, and most rnanage to deal with it deal with it effectively, but a wing official's concentration is best directed, totally forward for
EVERYONE'S best interest, to the field of play, rather than shared even just a little, with wondering whether coaches hovering behind him will take a step backwards (out of harm's way) when necessary in response to quick play action.
The view is just as good, and voice carries just as far from behind the restricted zone. A large part of this rule revision was in response to
SERIOUS INJURIES TO COACHES from unnecessary sideline collisions with officials.