I have seen a small percentage of truck campers unload their campers and leave in the truck. One issue is the amount of clearance between the tailgate opening and the typical bumpout. When the camper is loaded evenly, I have about 1/4" clearance. That makes loading a very slow and tedious process. I can get lucky and load within a few minutes or it can be a slow exhausting hour long process. The second issue I have is storage. I don't just travel for a few days or a week or so. My trips are typically cross country and last for months. I use the space between the camper and truck to store dirty laundry, extra boots, tools, a space heater, paper towels and toilet paper and all sorts of other items. There is no where convenient to put that stuff if I off load the camper. Finally there is the issue of convenience. It is great to spend the day traveling and still have access to the toilet, kitchen and be able to take an afternoon nap. I see lots of visitors in places like the Lamar Valley in Yellowstone. Many have big rigs outside of the park and are touring in their toads. They need to bring a sack lunch, remember to bring everything they might want and still have to use disgusting outhouses. Meanwhile I pull off the road, fix a nice lunch, sit in the dinette or outside on a lawn chair, use my own bathroom and then take an afternoon snooze.
