A. The manufacturers of shower arms have to meet certain pressure requirements, but water pressure is only present when the shower is turned on. Water normally flows through the shower arm and out through the shower head, not putting any real constant pressure inside the shower arm. Shower head shut-off valves are designed to drip some water through them so the hot and cold water will not mix at the shower valve itself, again relieving any real pressure build-up in the shower arm. So manufacturers can meet minimal pressure requirements and save money by using less material.