Because they cancel each other out? And even if you used a more powerful magnet on the front, the system still acts essentially like an internal force. Now if you made it external, and put a magnet on the front of the car, and an opposite-poled magnet on the rear of another car, and drove that car in front of the first car it might work, as long as you went fast enough.
I can't believe I'm seriously discussing a troll face science concept.
