I look at the various ways you can purchase a truck and find that the unitized construction, while thrifty and lightweight and perfectly okay for a car, would be a serious limitation for something as versatile as a truck chassis.
I'm thinking just of my old Toyota right now, but it applies across the entire range of pickups, even today.
The smallest U-haul used to be a Toyota cab and chassis, with a small box, about 8'. Remember those?
Going up a size, there were the many compact RVs based on compact pickups. There are larger models based on the larger American models too:
This is about as much as you can reasonably expect of a compact pickup chassis, except...
The Hilux was available as a 1-ton model, even with duallies and a flatbed (stakes optional)
I liked those and when I was shopping for a truck, I looked for one on the used market. No dice.
So while it might make sense in certain paradigms to consider a unitized chassis for trucks, in most paradigms it doesn't. There are too many sub-markets that use the truck as a starting point for their product. Granted they don't represent a large portion of the market but when did you last see any manufacturer willingly cede ANY sales to a competitor?
I can only think of two exceptions to this: the Honda Ridgeline and the Subaru Brat. The Brat was never considered a real truck, not even by Subaru. They were actually trying to horn in on compact truck sales with a truckified Loyale. And Honda will call their crossbred monstrosity a truck, but precious few people (Motor Trend and its dubious judgment notwithstanding) will look at that thing and call it a truck. It's a tall car with a lidless trunk.