Helping people improve their DPS will not aid in success in difficult mechanics. Also, while a challenging 4-man dungeon would be fun, how would it fit in? If rewards are equal to tomes, people will just spam those endlessly until they have everything they want, then complain about being bored, and if the rewards are less, people will call them useless. I get the impression from forums and interactions with people in game that very few would do challenging content for the sake of it.