I guess, and this is just my thoughts, not a fact, that the game itself creates a stereophonic or even surround sound effect by using monophonic sounds. Actually, there's just one engine in a certain place, be it the nose or multiplied into the wings. By turning your virtual head the engine sound appears to come from that direction where your nearest virtual ear is. You don't hear a stereophonic sound from a single engine IRL either, do you? Resonances and echoes are a totally different matter and they could and should be calculated according the cartoon pilot's head position.
Of course, stereo or multichannel recording in the cockpit of a real plane is logical in cases you can't move your head. In a movie it would work, because even though the pilot could turn his head around like an owl, the spectator - meaning you - would be sitting static on the back seat. When in 1st person position, the sound sources must follow the head movements and direction of view.
