If the person never decided to switch them. But if they lost a plate or they got stolen, they would have to apply for a new set. I wouldn't say it's rare to have the original black plates on a car, but with how often the cars have been switching hands in recent years, it is less common now a days to see them still on the car. Also some sellers would/will hold onto the plates as a memento, so another way they got separated from the car.
