Many of us prefer the ride and handling of a boxed frame that provides a stiffer chassis. It absolutely makes a difference, especially off road. I don't care if it is "stronger" by your definition of being able to carry more weight, I prefer a more rigid frame. Simple as that. I had an 06 F150 with a fully boxed frame and welded cross members and it felt so much more confident over uneven surfaces and off road compared to a Tundra, and compared to the previous generation of F150. I could pull the truck onto something and lift a tire, and all of the body lines still lined up just right. Doors opened and closed with ease. No squeaking or creaking of the cab or bed. Which is more than I can say for most every C channel truck I've owned.
Continual twisting of the cab off road is putting stress on every joint. Does it mean anything is going to fail, probably not. But in the long run it won't be as tight as it would have been on a more rigid chassis.
They probably use a boxed frame overseas because the truck is used more as a truck there. Hauling more weight and used off road more. Most Americans commute in their truck and drive to the grocery store and couldn't care less about what's going on underneath. It is a cost saving measure like the rear drum brakes. Nobody wants drum brakes off road, what a stupid argument (but one that most Americans have no clue about, and would accept as credible). Toyota will tell their fan boys and soon they will start bragging about how they have the only modern truck with drum brakes.