If a player has an issue with grades or needs classes, then a December signing date wouldn't help them much either, since most schools semesters end in January. They'd have to wait till February anyway.
And even so, those kids are also less likely to be committed in August as well, since Notre Dame would need to see at least quarter scores to allow them to commit at all.
I bet at least half, if not more, of the current commits would be able to sign in August if they had the opportunity.