The reason Giggs is the interim manager is very likely management higher up know his credentials are higher than those you have named.
PS: I completely understand your reasoning, and opinion. Makes a lot sense. Especially having Giggs as an assistant to a top manager.
That's what I want to see. Let someone like Van Gaal take over now. Get 3 odd years out of him and let the likes of Giggsy either be an assistant till then or better still be a manager somewhere else. Then, take a call at that time based on what transpires till then.