Because planes don't totally maneuver in a dogfight against other planes.
Plane vs Plane, this is simple. Two objects with the same low maneuverability. However, when fighting a dragon with maneuverability to the max. A plane stand no chance.
there are also non-heat seeking missles.
Computerized missles :D
The dragon would have to move quickly out of the way when the rocket is close. Totally possible too.
Also a think of a repeated thumping of locomotives nonstop.
Unless the bullets are computer guided. It would take a god to hit the same spot repeatly.
Aiming is also half done by computer.
Generally, computers aim to balistics. So once its shot, chance your direction.