It's the approach to the automobile as (nothing other than) a use item.
I also hear some people justifying their dirty cars by saying that their car is the one place where they can focus on themselves. Their commute is their me-time. So the car somehow represents a space where they're free from the expectation of cleanliness, etc., even though they're not messy people in general.
I'm sure most of us here are the complete opposite of those examples, but this is my philosophy on why I keep the space clean, beyond the hobby:
My cars are cleaner than my home because it's a much smaller area to maintain and it's more public facing, although my home isn't a war zone either like some people's cars. Also, you really don't want some of that crap in open containers getting heated up to 110+°F in the summer.
I've had coworkers who needed 5 minutes cleaning out their trash before we could get in and go to lunch. I've had to wait for food crumbs, fast food packaging, bottled soft drinks, canned soft drinks, exploded canned soft drinks, bath bombs, lotion, leaves, jackets, musical instruments, tools, sporting goods, boxes, stacks of paperwork, and more to be cleaned out. With one of my former coworkers, it was a damn scavenger hunt every time even though he would always say it was because of his teenage (at the time) kids. I never bought that.
In my book, it's a matter of personal pride and at least putting forth an effort to make a good impression.