Why name a starting QB if you don't have to? If I were the head coach (which thank God I'm not, I'm not qualified to be), I wouldn't name one until they trotted out for the first snap. Naming a starter publicly does nothing but narrow down the number of things the opponent's defense has to prepare for. Privately, it can help a team rally around one guy, but that announcement can be done privately, it doesn't have to be public.