Sometimes the space under/inside an RV sofa is used for other things. In my motorhome, the couch sits on a storage compartment accessible from the outside; a normal sofa would at the least need some rather significant modifications to work. Occasionally there are things besides storage there, like RV systems (furnace or water heater or whatever).
If it's basically just sitting on the floor, there's no inherent reason why any sofa couldn't equally well sit on the floor instead assuming you can get it in place.