Because the raid content would suffer without that story to underpin it, as I explained. If they don't care about the story, then what do the developers have to inspire the raid content to be more than a skeleton in a white room?

The story helps increase our excitement to fight these bosses and to continue playing the game. When there is a new patch or expansion, we are excited for the next chapter in the story. When we unlock these bosses, we are excited to fight them because we have all of the context and reason behind why we are fighting them and why the boss is so cool.