Cause when you have someone in the car and they ask what you run, you say "uh ... windows XP, and I'm proud of it." Then you get slapped. As opposed to saying "I have a custom interface with a touch screen", and people go "wow that's cool".
Using pre-made stuff with no modifications is like taking the bus instead of driving.
That - and Windows isn't exactly car-friendly (button size, font size, etc.). Try it in the car, instead of on your home computer where you have a mouse, a keyboard and all that.