Why are dealer plates coveted

They are not.

What makes u think they are?

Dealer license plates may be used on any motor vehicle, other than commercial cars, owned by the dealer (titled for resale in the dealership name). Dealer plates may be displayed on any motor vehicle owned by or lawfully in the possession of the dealer or the agent or employee of the dealer, & shall be displayed on no other motor vehicle. Dealer plates may be displayed on a motor vehicle when the vehicle is in transit from the dealer to the purchaser, when the vehicle is being demonstrated for sale or lease, or when the vehicle is being otherwise utilized by the dealer, a prospective purchaser, or a third party operating the vehicle with the permission of the dealer.

Dealer plates may NOT be used for delivery, hauling, transporting, or any commercial purpose.

Only the dealer can use them, the dealers has to keep track of them, & they r not even looked for by plate collectors.