Because we have a closed system when the charge goes off, directionality doesn't help. Ideal gas laws state that you could point the canister at the wall of the tube and still generate the same amount of force at the nose cone end. We don't do that because we don't want the flash or the crap going at the wall of our rocket, though. Might just hurt it....
Agreed. I learned from another member here when their L3 project suffered airframe damage from a hefty charge that blew a pvc charge holder out the side of the airframe. He can add more, or correct me, if I misunderstood the underlying issue.