I was reading a story in National Geographic about Somalia and it hit me. I know why the OT Stormtroopers are so terrible at their job. After the rise of the Galatic Empire, much of the galaxy fell into a state of dystopia, more or less. Money squandered in the war effort of the PT, and then in the war against the Rebel Alliance broke lots of the big businesses. The Galactic Empire became the galaxy's best employer.
They needed warm bodies and the disenfranchised civilians needed a way to vent their anger toward the Rebel Alliance and/or put bread on the table. So they start training as Stormtroopers. Their instuctors are equally disillusioned with the state of the Galactic Empire, so their training isn't up to snuff. Propaganda and witch hunts are what keep the general public afraid of Stormtroopers who're terrible shots.