New cars don't come with license plates. Used cars do, at least most of the time.
A dealership will often put its own temporary promotional "plate" on the cars it sells, presumably so you can let the world know how proud you are of where you bought your car. (Really, this is just free advertising for the dealership.) And for most car buyers, these temporary promotional plates get left on the car because, well, out of sight, out of mind.
I often hear buyers of used cars claim the car dealer never gave them the license plates for the car. They call the dealer who says the plates have been ordered from the DMV and the car buyer waits... and waits. But eventually the buyer is pulled over by the police for not having license plates.
If you're lucky, the permanent plates have been hiding under the dealer's temporary promotional plate all along and that nice police officer will let you know instead of giving you a ticket. Why didn't the dealer just tell you? Again, free advertising.
At this time, Cal. Vehicle Code section 4456 allows for a vehicle to be operated on the roadways without a license plate no more than 90 days after sale. After that you can get ticketed for not having a permanent license plate. Period. You will have to pay DMV directly to get the permanent plate, and demand reimbursement from the dealer for the fees you already paid on the purchase contract. If the dealer refuses, file a complaint with the investigations unit of the Cal. Dept. of Motor Vehicles. DMV doesn't like that.
In July 2016, Assembly Bill 516 passed, requiring dealers to put a temporary license plate (not promotional plate) on cars before they leave the lot. These will stay with the vehicle until the permanent plates are received. However, this law won't be phased in until January 2019.
In the meantime, check under the dealer's promotional plate for the real deal.