I'm not sure it's by "design" so much as catering to very specialized tastes is difficult/expensive. Add to that, the sort of people wanting "outdoor trip" vehicles are probably going to stress those vehicles a lot more than the person renting a car to drive 30 miles from the airport and back.

There's a reason renting Jeeps in Death Valley is something like $200/day--assuming you have your own insurance that will actually cover how you intend to use it.