I believe this is due to the fact the game uses regular distance calculation as you would in a normal car going on a trip. This is in the end hard-coded from the map system from what I understand. Speeding up specific vehicles has been suggested many times, and this has always been the response.
Understandable, its just kinda dumb where it takes over 15 hours for a plane to fly across the states where it should be 5 hours.