They are all interoperable, they all work exactly the same, and they all travel the same roads. If you know how to drive one, then you know how to drive all of them. The Macintosh, on the other hand, is more like a British racing car - cool, fast, but has the steering wheel on the right side. If one could run on a Mac, out of the box, PC software, then I would admit that a small Apple market share would be viable. The pace of technology change in the computer industry is quite incredible.
Products come and go, sometimes in weeks, but usually in months.
A personal computer user is perpetually making decisions about the viability of a technology. Cars, on the other hand, are stable in their basic functional technology. To use Don Norman's term, the affordances of cars have remained stable. Yes, the brakes are now anti-lock disk brakes, and the engine has a turbo charger, but you can still go out and buy a used car without these modern features and drive it on any road in North America. Spark plugs bought last year will still work this year.