Why can't all rooted phones be unlocked?
Most phones have separate AP (application processor) and BP (baseband processor AKA modem AKA radio). "Rooting" results in gaining control over the AP. The carrier "subsidy" lock is residing in BP. Most custom ROMs just include stock (i.e. still locked) BP image as a binary BLOB. Since the BP code is proprietary and never gets released to public.