As some have stated it has to do with the number of on-field coaches you're allowed to have. The OC might carry two titles depending on their background, Enos and Lashlee had both previously played QB. Other programs OCs are also the O-line coach, it's all about maximizing the value of each coach and the value of the staff.
That being said it doesn't make any sense to have Rumph and Banda coaching CBs and Safeties, fire Rumph and have Banda be the DBs coach if Manny insists on not firing him. Patke gets a very soft pass here because he coaches Strikers and Special Teams.