That’s not how plates are obtained. It has nothing to do with it. When buying a car from a dealership new, they submit all the paperwork, and your plates are sent to you within a month. If it’s used, it should already have a plate, but you can still get one from the DMV through online services in almost every state.
Not having a plate is usually someone avoiding tolls or red light cameras or some other petty crime thing. And to play devils advocate, I suppose it could have been stolen, too.