what about tyres? you can get a decent set of Michelins for my car for around £80 a corner and get 20,000+ miles no sweat even decent sports /tourers for a bike are around double that and you'll be replacing a rear in 5000 if you're lucky !
Apart from the aforementioned economies of scale, bike tyre technology (especially dual compound) is a lot more complicated than that for your average cage.
Also consider how a bike tyre is used, you've got a smaller contact patch, plus it's designed to lean over, which will probably result in more wear.
and you'll be replacing a rear in 5000 if you're lucky
That rather depends on how you ride.
I've just replaced a rear at 12,000 miles (Bridgstone T31) although I'd probably gone a bit further on that than was necessarily wise.
Generally I replace rears at around 9-10k miles and fronts at around 18k, but, whilst I may do some "spirited" riding, I'm not hooning it around and pushing the tyres to the limit.