I could see one reason why a HC wants to call plays when he came up through the coaching ranks doing so as an OC. He has his system. he knows what he wants and what his strategy is for doing what he wants to do.
OCs come and go. If Frost calls the plays and is that involved with the system, then if Lubick gets a HC gig a few years from now, the system wills till mostly stay the same with a new guy coming in.