Although the payload capacity of an American truck is often double that. It’s pretty easy to hit 1500lbs with 10 bags of cement, a person, and some gear. I’m assuming Japan has other solutions for hauling that are pretty common. I’m guessing there are some pretty big differences in their construction workflows and logistics that make key cars practical.
IMHO, contractor’s work trucks aren’t the problem is the US. The bigger problem is that most of those vehicles are sold to private citizens who think the space is convenient for an occasional camping trip or an epic Costco run.