New cars are generally not shipped by piggy-back trailer, except as the final stage of delivery. Inside the US, they are shipped by rail. As the car must be certified as damage-free during shipping, including the absense of any paint damage whatsoever (including fallout), the cost is more than you may think. All of the seats are wrapped in plastic, large sections of the carpet are covered in plastic, and the paint is protected by either a special plastic adhesive or a chemical compound. All of this must be removed with the car prepped and detailed for sale on the dealer end. During shipment, the car must have an empty gas tank, which sometimes presents issues when removing it from the rail. Dead batteries and lost keys (or locked-in keys) are another problem. Then there's the cost of insuring all those cars in a constant state of shipment...