Because the thing the front magnet I attached too would have to be heavier than the car for the car to move, at which point the car moving forward wouldn’t cause the front magnet to move forward because the thing it’s attached to is heavier than the car. So you’ve got 2 failure states.
1. The car is heavier than the front magnets housing thing, in which case the front magnet gets pulled back into the car and then stops and nothing moves.
2. The car is lighter than the front magnets housing thing, in which case the car moves forward, it’s magnet hits the front magnet, stops, and nothing moves.
It's because both magnets are "pulling" on each other with the same amount of force, and because they are both attached to the car the net force is 0, meaning no movement.
1. The car is heavier than the front magnets housing thing, in which case the front magnet gets pulled back into the car and then stops and nothing moves.
2. The car is lighter than the front magnets housing thing, in which case the car moves forward, it’s magnet hits the front magnet, stops, and nothing moves.