Because rockets are built for a specific role, that is boss DPS. That is the ONLY reason you use a rocket launcher. If you use a rocket launcher for anything other than Boss DPS you are not using the weapon correctly as LMG's fill up the same slot and do it better. Saying Dragon's Breath a Rocket Launcher is good for Mob killing puts it in direct competition with other weapons that fill that same role, LMGs. Which if fails miserably compared to them.