I have The Dirtiest Car in the World.
Around here, when you see cars covered in dirt, they are usually SUVs, and you can tell that they are dirty because they've been to the mountains.
I have a little white Mazda. My car is dirty because it's been to the airport.
When I travel, I park near the airport, and my car gets covered in dirt and grit (warning: don't breathe near airports!). I've been to the airport several times this summer. In between, a few days go by as I settle back in, and then I forget the car is dirty, and then I realize I'm leaving for another trip soon, so why bother getting it washed?
The dirt does not look like mountain dirt; it does not look like it was kicked up by dirt roads. It looks like airplane exhaust fell on it.
I'm leaving again for another trip soon, so why wash it now?