Some of those whose dogs 'don't bark' are either deaf or have become so attuned to it they just don't notice it. "Our Fluffy doesn't bark when we leave her alone in the camper". How do you know? The folks parked next to you who are trying to relax or sleep or have an outside lunch might disagree with you.
Of course not all dogs bark, but some will bark each time a vehicle drives by their site. These pups are not barking because they enjoy it, they bark out of boredom, fear wanting attention and any number of reasons.
Good friend we used to camp with, now with Alzheimer's, had a pup that barked constantly. I asked him one day of the pup was OK and he had not even heard the pup barking. We certainly had been hearing it for quite a while