Re: It's a f'in car!
It's not rust anymore. Other reasons are:
"Oh, that part is not available any more, and there's no alternative", and then finding out that it's a common failure once the car is about 5 years old. They'll be none in the breakers yards either. "They're like gold dust. I'll call you if I get one in, but it'll be expensive because there's high demand, and I don't often see one of those with it not broken".
Or special tools necessary for routine jobs on higher mileage vehicles that are too expensive for the small garages to buy, meaning that you have to pay main dealer labour prices or get rid of the vehicle!
I have a ~25 year old MPV that's got to the "parts not available" state (but I'm not complaining about this, it's about time this vehicle was taken off the road, if only for emissions), but I reckon that it will be significantly less than that for any vehicle made today.