It's only speculation by me but I feel like the lack of better rewards for things like achievement tokens, variant dungeons or any of the hundred of currencies we collect is because they don't want anything that they could charge money for in the mog shop available in the game for in game currency. So the really good items are either limited time events which then become mog shop items, or are just mog shop items. If you put items, even just glamour items, in game stores then it detracts from what people will spend money on in the mog shop.