Sitting for 8-12 hours a day in a cramped position is bad for your back, neck, shoulders, hips. Name a joint and it's gonna get messed up eventually. There's also a risk of blood clots and generally anything that can be caused by poor circulation, especially in your legs. Add to that poor sleep from sleeping in your cab or possibly a crappy motel if you're lucky, not getting enough sleep, rarely being able to exercise or eat well, and obesity and muscle degeneration start to become pretty serious risks, too.
You can compensate for all those things, and some drivers do. Taking walks on their breaks, doing calesthenics, bringing good food from home, etc. Many of them don't, just as many other people don't take care of themselves and are fat and sedentary.
The amount of sitting a software dev does is comparable if not more to most truck drivers, I'd guess.
I can get out of my office chair, stretch, and then return back to my seat in a minute as much as I'd like. An 18 wheeler has to exit the highway, find a place to park that accommodates large trucks, put on the brakes, Climb out, walk around, and then go back to the highway. All while not being paid a salary but being paid by the mile.
