Almost every passenger car is built as a single structural shell, yet many trucks and some large off-road vehicles retain a separate frame. The older method persists because it does specific things the newer one does not.
How the two constructions differ
A unitary body uses the whole shell as the structure, with panels, floor and pillars all contributing to stiffness.
Body-on-frame keeps a separate ladder chassis carrying the powertrain, suspension and load, with the body bolted on through rubber mounts.
In the second arrangement the body carries occupants rather than loads, which changes what each part has to be designed for.
Why load handling favours the frame
Towing and payload apply concentrated forces at specific points, and a ladder frame provides direct load paths to spread them.
Repeated heavy loading also fatigues structure, and a frame can be built with the section depth to tolerate that without the whole vehicle becoming enormously heavy.
Off-road use adds twisting loads as wheels rise and fall independently, and rubber body mounts absorb some of that flexing before it reaches the cabin.
What it does for manufacturing
The same frame can carry different bodies, so a pickup, a cab-chassis and a large wagon can share their underlying structure.
Commercial operators can also fit specialist bodies to a supplied chassis, which is impractical with unitary construction.
Repair is simpler too, since a damaged frame section can sometimes be replaced or straightened where a bent monocoque may write the vehicle off.
What it costs
Carrying both a frame and a body means duplicated structure and considerable extra weight, with consequences for consumption and braking.
The high frame raises the floor, which either raises the whole vehicle or reduces interior height, and lifts the centre of gravity.
Rubber mounts that isolate vibration also allow relative movement between body and frame, which makes precise handling harder to achieve.
Why unitary construction won elsewhere
A monocoque is lighter and stiffer for a given amount of material, and stiffness is what makes suspension tuning predictable.
It also allows crash energy to be managed through engineered deformation across the whole structure rather than through a rigid rail.
Modern crash requirements are met more readily this way, which is why the separate frame has retreated to vehicles whose working duty justifies its penalties.
Several large off-road models have moved to unitary construction with reinforced subframes, keeping much of the load capability while recovering the weight and refinement a ladder chassis costs.