Overpowered. That's why. Bungie will never say this, but they are trying to avoid people cheesing (in their eyes). Any boss with decent mechanics and lower health would just be ghorn'ed off blackhammer'ed to death. They have alluded to this but know they can't have an open discussion (see similar threads as this one, people are pretty crazy - not all). I for one am ok with this. I see your point, but they are just trying to fix the game. Or maybe they just are trying to force everyone to buy more DLC to get the most powerful guns.... We'll know in a year.