Almost every passenger car uses a single welded structure, yet pickups and the largest SUVs still bolt a body onto a separate frame. The reasons are structural and commercial rather than conservative.
The two approaches carry loads differently
A unibody uses the whole shell, including the roof, pillars and floor, as one load-bearing structure. Stiffness comes from the assembly as a whole.
A body-on-frame vehicle puts the ladder frame in charge of carrying loads, with the body attached through rubber mounts and contributing comparatively little strength.
That separation means a heavy towing or payload input is fed into the frame and never has to pass through the passenger compartment structure.
Frames tolerate heavy point loads
A towing hitch, a fifth-wheel coupling or a slide-in camper applies concentrated force at specific points, often well behind the rear axle.
A ladder frame accepts that force through its rails directly, and reinforcing it means adding section to a member rather than redesigning a shell.
A unibody can be engineered to tow heavily, but doing so requires reinforcement spread through the structure, which becomes expensive at the loads commercial buyers expect.
Manufacturing flexibility is the commercial argument
One frame can support a pickup with several cab and bed configurations, a chassis cab for a commercial body builder, and a large SUV.
Each variant is a different superstructure on a shared foundation, which keeps tooling costs down across a range that would otherwise be uneconomic.
It also allows outside converters to build ambulances, tippers and specialist vehicles on a bare chassis, a market that a unibody cannot serve at all.
The costs are weight and refinement
Two structures weigh more than one, and the extra mass hurts consumption, braking and handling in ordinary driving where the towing capacity is unused.
Body mounts isolate some vibration but also allow slight relative movement between body and frame, which is felt as a lack of tautness over rough surfaces.
Handling suffers as well, since a taller vehicle with a body flexing on mounts responds less precisely than one rigid structure would.
Where unibody has taken over
Family-oriented SUVs moved to unibody construction decades ago because their owners tow rarely, value ride and refinement, and benefit from the lower weight.
Electrification pushes further in that direction, since a structural battery pack under the floor provides enormous stiffness and duplicates what a frame does.
The frame is likely to persist where the vehicle is genuinely a tool, sold on payload and towing figures that buyers compare directly before purchase.