Nowhere to go

Some places stop taking reservations in the fall and become first-come-first-serve - and their website shows "full" on their campground map.
But if you read the alerts, they will tell you so.
Other sites, like forest service campgrounds simply close and they too stop taking reservations.

First, you are limiting yourself to state park campgrounds. There is nothing wrong with them, but there is a lot more out there.

Federal campgrounds (USFS, COE, BOR) are a great option.
USFS campgrounds are usually open only during peak season with some being open during the shoulder season on a FCFS basis. Unfortunately, with the extensive fires in recent years, they are very likely to close earlier. And late snows can postpone their openings until as late as July. So it tends to change yearly.

Many of the COE campgrounds are in the foothills. That means they are open year round. During peak season, they are very hot (think valley weather in the summer) and very crowded due to the water sports people (boaters, skiers, jet skiers, etc). In off season, they can be great weatherwise and are far less crowded. I have never had a problem getting a reservation (or even just heading out without one) between October and April. You should check online and make sure the campground you choose doesn't have a closure due to maintenance, etc, as that can happen.

BOR campgrounds are fewer but follow the COE campgrounds.

For me, the most difficult time to camp is August and September. The weather is too hot for me in the foothills and valley areas, and the mountain campgrounds are getting hit by fires and closing.

Here in the south east, the state parks that you want to go to stay pretty booked. After labor day, some of the campgrounds on lakes you can find some sights. The Campgrounds in the mountains stay pretty booked.

You have to book way out, or luck into a cancelation.

There are some state parks that always have availability, but they are in the flats in the middle of GA, and not really an appealing place to go, no beach, no mountains, no lake.

P.S. Check your PMs, Lightning55. I shared a secret spot with you.

We're headed to the desert tonight. It's actually going to be cooler there than home.

It's wide open, free, and doesn't require reservations.

My point is to get creative, because there are places in CA to camp. This isn't to minimize your problem though, because finding camping spots is definitely a crazy byproduct of the pandemic.

I don't want to give away any of my secret spots, but I've noticed that a lot of seaside communities are starting to open up funky parcels of land for camping. They aren't cheap and they're typically dry camping (no hookups), but they're definitely springing up.

We went out last weekend and completely overpaid $72/night to be sandwiched into a gravel lot downtown at the beach. There were vacancies though. The only reason we went was because our close friends were going and we love them exactly $72 worth, but not one cent more. If it had been $73, we probably would have balked, and $75 would have been a deal breaker.