The reason for replacement is basically wear on the U-joint needle bearings. They are not a part thats known for lasting forever, on any vehicle. Like almost all cars, these are no longer held in place by circlips but by injected plastic. They don't use grease fittings either as there's not enough room for zerk fittings. Something's got to wear in this articulated swing arm drive system and the u-joints are the most preferred and easiest to replace