How do you know they don't? You wouldn't be able to tell which way they are going unless one of the ships is not manuevering. As soon as you try to attack an enemy from below, the enemy would either point its bow towards you, or escape. Their methods of propulsion allow them to go forward/backward, so that's where the torpedo lauchers are, and you would only face an enemy with your front or back. In All Good Things Enterprise does attack from below, but that was because she was cloaked and was able to achieve surprise before Klingons could turn towards her.