It’s explicitly stated in A New Hope that they were shooting to miss when the main characters were escaping the Death Star because a tracker had been planted on the Millennium Falcon and they planned to follow it and destroy whichever planet was hosting the rebel base. You can’t do that if you don’t let them escape. Every other time they’re in battle, stormtroopers are shown to be devastatingly effective except when a small garrison is overwhelmed by a vasty larger number of ewoks.