Traditionally we have beds that fit between the wheel wells.
They could be made in a press brake and you only needed a stamping die big enough for the fender around the wheel.
A mechanical die press 8' long was very, very expensive and the number of reject parts would be high.
With a step in front of the wheel farmers could stand high enough to shovel out the bed.
And there are no wheel wells (nor the inside corners) for dirt or horse poop to get caught in.
But the family farm was getting rarer post WWII.
Vehicles in general became more stylized in the jet and rocket ages.
Flowing body lines were the future, and form took a front seat to function.
Huge hydraulic presses sat idle after the war.
Millions of young men weren't going back to the farms they were born on.
By the '60's we had
styleside pickups, and ute's like the Ranchero that took a Torino's body lines all the way to the tail lights.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.