In a manner of speaking companies such as MMC paint themselves into a corner with generic and service parts readily available elsewhere. They are obliged to stock them but likely have poor sales due to many knowing where else to buy at realistic prices. Consequently they have huge markups to cover their stocking overheads.
When I was in the motorcycle parts business I tried to avoid marking up low turnover parts too much by writing them off against tax a.s.a.p. That way when I did sell one I would still make a fair clear profit. Re-stocking was another matter of course.