More to the point, I’m asking why you think vans are a superior option to trucks in this role.
Things like the Ford Transit or Mercedes Metris are built like cars with unibody construction and car suspension. That’s fine as far as it goes, but they’re not clearly superior to a pickup in the same size range. The Transit Connect is roughly comparable to the Ford Maverick, and I believe even uses the same engine. Comparing fuel economy:
The van has a smidge better city millage, but the truck has a fair bit better highway. That’s likely because the truck weighs a bit more (which matters more in the city) but has better aerodynamics (which predominantes for highway cruising).
Why would a truck have better aerodynamics? Frontal cross section. Vans tend to be as tall as a truck, but also sit lower like a sedan. They would tend to have better drag coefficient, but their frontal cross section is worse. By way of contrast, a motorcycle with a rider has a drag coefficient that makes a truck look good, but has a relatively tiny frontal cross section.
And here we might have our answer to why Americans may prefer trucks in this role: more highway driving.
Past the Transit and Metris size, yes, vans are built like trucks. A Ford E350 van is an F350 frame. Same with the Chevy Express and the Silveroto. The Mercedes Sprinter doesn’t have a direct pickup analog, but it is sold as a cab chassis for customization.
More specifically, they tend to be body-on-frame construction. They have to in order to support higher weight capacity. The Sprinter is a bit of a hybrid between unibody and body-on-frame–hence allowing a customizable back end–but anything bigger than that has to be built like a truck regardless of what’s on the backside. Unibody car-like frames don’t cut it.