Can't see how you would write a script to handle the endless variations of each trip back and forth, and that's assuming that a certain trade route stays viable over a period of time, something that is designed to change with players' interactions.
Take just a simple process of leaving a station, flying out, and turning back to land again, FD has issues with their NPCs doing just that, and they have the advantage of working within the game code! How will you do better than current NPCs with very little way to get feedback on your surroundings?