I thought the USMC had decided that ops from the smaller decks were to be VTO and VL - not much room for anything else. Where there's space, the flat TO is fine. USN will have proper carriers and no need for ramps.
The snag with a demountable ramp may be the fact that it's curved. If you could lower it, you'd be left with a big dip in the deck. And what would you do with that deck space if it's going to be needed again for the next launch. Mind you, International Rescue had a retractable ramp for Thunderbird 2, so it must be possible.