In a truck you can't just... drive anywhere. There are designated legal routes that you can't deviate from under most circumstances. The vast majority of food you have access to as a trucker is gas station/truck stop food and fast food attached to truck stops. You have limited cooking options in a truck, maybe a small waffle iron, rice cooker, or microwave. Most won't have room or power for something like a refrigerator (some might, it depends) so keeping non-dry food isn't really an option.

First, your work day is almost completely sedentary. Second, you are not near friends, gyms, fields, or other systems which promote healthy activity outside of work. While diet is important for all of us, it is especially important in this circumstance.

"Healthy foods" are going to be largely inaccessible through long sparsely-populated stretches of the country. Even where they are available - in markets, fruit stands, etc. pulling your huge truck off the road to park is expensive (in terms of time) and difficult (in terms of space). Truck stops have the infrastructure for your truck. But their food selection is mostly junk.

They generally don't have access to a kitchen or a large refrigerator, so meals tend to either be from a restaurant (where most restaurants, good and bad, are arguably not healthy unless you severely portion-limit) or something that can be reheated in a microwave or portable air fryer. In combination, this results in a not quite healthy diet.

thanks to all for the good answers.