I'm sure it's because of the lack of assigned seating. Fake a disability and you get to pre-board, and get your pick of seats ahead of the crowd without buying an upgraded position. Every flight I see several wheelchair attendants leaving with empty chairs after the flight unloads, as these passengers that needed wheelchairs to get down the jetway are somehow able to walk back up completely unassisted at the end of the flight. It's a jetway miracle!