Icons for planes are there because planes become a dot at ranges that, in real life, should easily be identified as a plane (and often plane type) and easily determinable which way it's flying and how fast it is moving towards or away from you. While those skills certainly varied from pilot to pilot, the icon system gives a rough approximation of the pilot's tacit knowledge of the situation. Buildings, OTOH, don't move, are visible from far enough away to do anything you need to do, and you can learn to estimate proper range with experience, just like in real life.
Regards,
Hammer