I think it's two separate but each probably sufficient reasons: 1) Linus doesn't want to (unless his stance has changed, haven't seen it). 2) It would be almost

I think it's two separate but each probably sufficient reasons: 1) Linus doesn't want to (unless his stance has changed, haven't seen it). 2) It would be almost impossible because of contributor copyright/license reasons, as you explained. So I'd really say it's both.