I think it should be restated again, that this is a relatively under-developed (compared to the rest of the heavily visited, and more developed Hawaiian islands) island. The fleet here is finite and extremely limited. This is not a large parking garage like structure, with rows of cars, and dozens more to pull from inventory. Just look at it on any mapping site.
Additional food for thought:
There are no BMW, Audi, Jaguar, Mercedes, Infiniti, etc dealerships on the islands. AKA, there are no ways to get cars serviced under warranty by authorized repair centers. As a result, you will find an extremely small amount of these brands on the roads. So even if you used a rental service like Turo Car Rental, you can see there are no exotic cars to rent. And of course, I doubt Hertz, National, or any car company has an actual exotic fleet. I don't even know if there were Cadillac or Lincolns on the road.
As far as the dirt in the vehicles; the island has a rusty red dirt will stain practically anything it seems. Even after you have returned home, washed your clothes multiple times, the stains will still be there. I would wager that tourists like us, are just tracking this red dirt in the vehicles, and any car with a light color interior will show the stain horribly. As a result, I assume National Car Rental fights a daily battle with vehicles stained red like crazy inside.