People generally let the dealer sock them with an additional $500 to cover car registration for them (so they don't have to go to the DMV themselves). When this happens, the dealer gets to install the plate for the customer, and they'll throw on the dealer frame at the same time.
After this point, customers don't have much reason to remove the plate or frame unless they really hate the ad, happen to be self-installing a rearview camera or are selling the car.
One explanation I heard is that companies give them for free, so most people are happy to take them and don't bother with removing the ad.