Developers did this due to the game's limitations because spawning a pilot could take more operating and graphic memory, which was low back in 2004, the year GTA San Andreas has been released. All of the aircraft are set to spawn without a pilot. This can be proven by looking into the handling files of the game.
It has been noted that the planes have no pilot, even when in mid-flight. The strange thing about these crashes is that upon inspection of the wrecked airplanes, there does not seem to be any pilot. Other times, the airplanes may crash head-on into the ground near the player, often resulting in the death of the player or destruction of their vehicle. These airplanes seem to be oblivious to their surroundings and may crash head-on into objects, even though there may be enough time for them to maneuver out of the way. Around the state, airplanes can be regularly seen crashing into tall objects.
