Because when they pull you over and sit in their car for a minute before getting out, they are calling in your license plate number and make/model/color of the car. The dispatcher types it into their system and radios back to the officer with the owners name, address, whether the registration is valid, and whether the owner has any warrants out for them. They will automatically have all the information about the registration before they even get out of their car.

This is based on my personal experience as a campus police dispatcher in college more than 40 years ago. I’m sure by now, they have data terminals in the car they can type it in and look it up themselves.