In the first movie civilisation hadn't completely collapsed yet so the cars and bikes look in pretty good nick - which is how they should have looked. It's only in the second film that things get really post apocalyptic & rusty since at that point civilisation was all but gone. (I watched both films back to back last night)
They're basically a cult that worships vehicles and being "shiny", I think they'd have rather well-maintained vehicles.
The cars in the first movie were all in pretty good condition, from memory.