It is bizarre. Here in California you didn't used you buy a car with plates at all. Then they started to require paper temporary ones.
Why they don't just have plates fitted on the lot I don't know. I can't see how it saves any time, the dealer would just have to have the plate ready for each car like in most of the world. It seems that the simplest solution was too hard to implement.
in CA, new cars are issued permanent plates when they are registered. once they're registered, they're considered used cars, so dealers don't register them while they sit on the lot (they also don't own them before they're sold, usually a bank or the OEM does). the temp plate system that was just introduced worked fine in states with lower car sales but probably took a while to roll out here.
it's a combination of outdated laws, outdated technology, and massive amount of car sales. it's finally catching up.
In many states you get a temporary plate from the dealer that is good for 30 days or so, and receive your permanent (or annual) plate from the state in the mail.
If you buy a used car from a private seller you have to go to the motor vehicle bureau and transfer the title and register the car yourself. If you buy from a dealer they handle that for you (and charge you a few hundred dollars as a "document processing fee").
In some cases you can transfer the plate from your old car to your new car, but that's not always possible. There doesn't seem to be a lot of rhyme or reason to it.
Why they don't just have plates fitted on the lot I don't know. I can't see how it saves any time, the dealer would just have to have the plate ready for each car like in most of the world. It seems that the simplest solution was too hard to implement.