It’s pretty common to have unlicensed off road vehicles, especially in the mountain west. Farmers and ranchers often have at least one of these. There’s plenty of recreational users as well.
That doesn’t mean that this isn’t true in a technical sense. It’s correct that it isn’t feasible for the majority of the population.
You’ll sometimes also see small communities with private roads that allow unlicensed vehicles, such as retirement communities, but they often have their own standards for what is allowed.
If you are driving off-road, or completely on private property, you're not really driving the vehicle to "go somewhere" or commute or transport people/goods.
It isn't really feasible to use a vehicle for actual transportation without using public roads, at least in these United States.
So what possible cause or reason would any law enforcement have, for going into a vehicle like that and searching it? I mean, compared to someone driving on a public road and "going somewhere" while "carrying stuff" in there? Nearly none, right?