Every force has an equal and opposite reaction.

When you are holding the magnet in front of the car you are pulling the car along through the magnetic attraction between the magnets, and the car is pulling the magnet back towards itself with equal force. However, you can just pull the magnet away as the car gets closer. When you do so you are transferring the force the car is imparting on you into the ground you are standing on.

If you were to now get on top of the car and hold the magnet in front of it the magnet you are holding would be pulled towards the car, and the car towards it. However, since you are on top of the car, instead of the force the car is imparting on you going into the ground (allowing you to keep moving the magnet away from the car) it would go back into the car. This force going back into the car is identical to the force the magnet you are holding is imparting on the car. As such they cancel each other out and the car does not move.

If you repeat the above, but replace the magnet with a rope, it’s a lot more clear why it doesn’t work. You can’t pull on a rope while sitting on a car and expect the car to move. The magnetic force is the rope, and you pulling on the magnet is the same as pulling on the rope.