Because as DC while you are leading a unit the HC is the final word on all matters, if a player is not getting it done he has more to worry about with the HC than the coordinator.
While you are
LEADING a unit...
Is said unit filled with men?
Is Wade actually leading the unit?
Does he hold those men accountable and make decisions and recommendations on their status with the unit?
Do they play for him, who is not their immediate supervisor, but the manager of all the supervisors of that side of the ball?
Does Phillips create a defensive game plan, and assign tasks and grade out the players as to their execution of that game plan after the game?
I challenge your one-size-fits-all assertion the DC doesn't have the ability to move players from starters to back-ups depending on their success.
I have no issue with Phillips not being a man who was a successful head coach. But I also see him posting 13 wins in a season as that head coach. And if Jesse Holley is to be believed, I see Garrett scuttling the offense to get the head coach job. Which I have no issue with if Phillips was not aware enough to hold Garrett accountable.
Although, to be fair, when Jerry essentially names Garrett the co-head coach and created a house divided by allowing Garrett autonomy, Wade was on the way out at that point and all the leader of men arguments went with it.