If by flatbed you mean pickup trucks; My guess would be it’s a holdover from when America was a lot more rural and much more of the economy was agricultural. Pickups are great for work here you need to move lots of heavy, dirty stuff like dirt or bricks or cinderblocks or whatever random construction materials. Just throw them in the back and go. Same goes for all kinds of heavy tools. Just shove them in the back. The open bed means you can access the cargo from any direction and there’s no limit on how tall your cargo can be. You can fit it out with all kinds of accessory racks to carry different kinds of tools and equipment. And pick-up trucks, at least historically, were pretty rugged and well suited for operating on poorly maintained dirt or gravel roads.

A lot of people in the US who use trucks for work do use various kinds of transit vans, they’re very common for all kinds of tradesmen. But pickup trucks have become sort of a luxury status vehicle so there are tons of wildly oversized pickups that don’t actually get used for anything. A lot of them are made with a crew cab so you can seat 4-5 people, but they only have a 4 or 6 foot bed so you can barely haul anything.