US manufacturers are smart not to waste their time and money making campers for trucks that almost no one here has for personal use. Sure flatbeds are everywhere as work trucks but they are a tiny percentage of personal trucks.
Bed delete or removal is usually out of the question too. The whole point of a truck camper is that you can easily drop it on and lift it off of your pickup, and still use the truck for other things. If it’s permanent, that’s a “motorhome”.
I’m not against flatbeds or flatbed campers. They’re great. It’s just a matter of building what will sell if you want to stay in business.