To add to the "why" question, several factors do play a part in the mechanics against the plane. Most of the time, with smaller aircraft, weather will be a primary effect (although even larger aircraft can be affected by this). When weather is applied, and wind is greater than 0, it will apply a force against the aircraft. In addition, when taking off or landing, we flight into the wind, or as close to it as possible based on available runway direction. This adds to the forces against a plane which will cause the plane to "steer" in the direction the wind is blowing. This also occurs on a taxiway (if you were in an open field, without any obstructions, and you were to let go of the joystick while moving, you would notice that the aircraft will turn in the direction of the wind and then it would stay straight once it's in the same direction as the wind.
Another factor that will play a role is the gyroscopic effect from the engine, in most prop-based aircraft they will tend to turn clockwise because the force from the engine causes this to occur. This is generally noticed when in the air but with enough force, could be noticed on the ground as well. A dual-engine prop aircraft will sometimes have one prop turn clockwise and the other turn counter-clockwise to compensate.
Finally, you will also notice terrain is not level, and neither are taxiways and runways. There are bumps and curves, and dips. This all adds to the complexity when trying to move in a straight line, just as it does at a real-life airport.
Hopefully this helps you get a better understanding of the forces that are applied in the sim. X-Plane is one of the only sims that I have found that does a fantastic job of simulating physics and forces against objects.
