Why aren't Rogallo wings or parafoils used in aircraft with fuselages?
Larger aircraft: Metallic wings

How much fuel could you store in the Rogallo wings if you need some?


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The wings provide lift, but not only. In large aircraft they usually receive thrust from the engines, in order to transfer it to the fuselage, they are also fitted with hyper-sustentation devices and aerobrakes. And finally they can transmit a good part of the aircraft weight to the outer landing gears when the aircraft is on the ground.


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The latter functions are not compatible with a flexible structure. Only metallic or composite wings are used. They can be adjusted for an accurate envelope with optimized performances.



Smaller aircraft: Wood and canvas or metallic wings

The structure can be stiffened, this is commonly done, and leads to the concept of wood and canvas, as the DR-400 and many other GA training aircraft (all first aircraft were built this way):



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However these airframes don't scale well, and are not compatible with fast GA aircraft, leaving alone airliners speed range. Larger and faster aircraft, which anyway must protect their wing tanks, use metallic wings.



Ultralights and paragliders: Only categories with Rogallo wings

The advantages provided by flexible wings (easy storage, light weight, low cost) is a significant choice criteria for use in ultralights and paragliders, and of course for parachutes. These advantages overcome their limited aerodynamic performances, which is of lesser consideration for vehicles that are not specially aerodynamically designed.


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To summarize: The flexible wings are more appropriate for smaller and slower aircraft, in particular those not requiring wing tanks, wing control surfaces, or wing landing gears. Namely ultralights.