Really? That's an interesting assertion. Pray tell, why is there a cargo door and floor on a KC-135? And a KC-10?
For the same reason that the KC-46A needs them and the MRTT doesn't, no? All of the former are unable to fully utilise their underfloor cargo spaces because of the need to fit auxiliary fuel tanks. Without the need for auxiliary tanks, the MRTT is able to utilise all of its underfloor cargo space, and so doesn't
need a main deck cargo floor or cargo door.
A main floor cargo deck and door aren't desirable features in themselves, it's just that the other platforms you name
require them due to a lack of available cargo space elsewhere.