Quite frankly, the state of California couldn't care less if you have comprehensive and collision coverage on your vehicle. What they are concerned about, and why liability coverage is mandatory, is that they want you to be financially responsible and able to cover the costs if you cause damage to someone else's car and/or hurt them physically. They require every car owner to have liability because they want to make sure the other party doesn't suffer financially from the mistakes of other drivers. Nowadays the mandatory liability limits are too low as they haven't been increased in decades in most states but that is why that is a requirement.
Who does require comprehensive and collision is lenders who give you the money you need to buy a car, used or new. They are the ones that want your vehicle insured so that if something happens to it they are not out a ton of money if you quit paying your loan and if they have to repossess the car it's not worth much because it has been damaged or stolen. The lenders don't allow "self-insurance" for this because they want an insurance company saying that they will cover damage or theft of the car and don't trust some individual saying they have enough money to take care of this themselves.