Given how much faster ans easily front chainrings wear out and get 'chipped' at every time you shift then I imagine with the cassette, given you would shift across it a lot more than you would a chainset, is not going to last too long. Possible quite a good reason why they are not prominent in Shimano's line up and Ti/Steel are used instead. I think it's the shifting that punishes them far more than the actual riding on them.