As for the "why": I'd love to learn about all of the historical and regulatory incentives here. From what I've managed to cobble together, this is partly due to CAFE standards[1], partly due to Section 179 of the US tax code[2], and possibly(??) due to contractor demand[3].
[1]: In 2006, the NHTSA adopted a "footprint" approach to CAFE standards, where fuel efficiency standards are set based on the vehicle's wheelbase multiplied by its average track width. I haven't dug into why this is the case, but people seem to agree that this results in less stringent fuel economy targets for larger-sized vehicles. See https://en.wikipedia.org/wiki/Corporate_average_fuel_economy
[2] A key piece of U.S. tax code, Section 179 allows businesses to deduct the full purchase price of qualifying equipment up to $1M(!) For trucks, the vehicle must have a Gross Vehicle Weight Rating (GVWR) over 6000 pounds to qualify for the Section 179 deduction. This incentivizes businesses to purchase larger, heavier trucks. See https://en.wikipedia.org/wiki/Section_179_depreciation_deduc...
[3] A friend told me (citation needed!) that the ability for a truck bed to carry 4x8-foot drywall sheets is often advertised as a selling point (is this true?). I suspect a lot of this is just the US market lusting after "bigger is better", but I'd love to hear anything others know about this.
Anyone have details they could add to this?