You're thinking of a torpedo as a bullet. Although some of the games have treated torpedos that way, the "real world" definition of a torpedo is something we need to consider. A torpedo is a limited range, self-propelled, SELF-GUIDED device. Normally, the payload on a torpedo is a warhead, though in real life, as in the case of Trek, other payloads can be loaded into a torpedo casing.
Think of the shots from "Hunt for Red October" as a really good example of what we SHOULD be thinking when we consider Trek-nology torpedos. They can't just weave around constantly, because they have limited fuel reservoirs... but they certainly can, and should, be able to track onto single target's location and perform course-corrections as they approach the target.
I always assumed that the "light" we see from a torpedo isn't the torpedo itself, but rather the propulsion exhaust. It's thrusting, and steering, all the way to the target... going faster and faster (and becoming harder to steer in the process) as it moves further and further away from the launch site.
There's no reason to treat a "torpedo" as a "dumbfire" weapon. BY DEFINITION, a torpedo is a guided device.