I would guess it's because cranks, gear sets and cylinders are consumables and have a limited lifespan, but the crankcases don't have many issues that does them in. I bought a new set of crankcases for my H1 years ago, ($283 NZ) because the ones I had were destroyed from a crash that put the front wheel into the case front mount web and smashed it. But that sort of thing isn't a common problem.
_________________ 1974 Kawasaki H2B 750, 1981 Yamaha XV750 Cafe, 1986 Kawasaki KDX200, 2003 Honda XR100, 2004 SDG140. 2006 Ninja 500R Turbo intercooled fuel injected.
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