The game is a business with a subscription model. Make it harder and you will alienate a lot of people. Specially those who work and don't have time to practice for a week to clear a dungeon. As much as raiders want to they are a tiny minority.

Want harder content? Do savages and ultimate's. Then complain all you want if its not hard enough.

The game is more of a dancing game than a fighting game. Is all about memorization of a dance. If it was a fighting game, then bosses would have an adapting AI. Instead they do the same thing over. Thus a dancing game.