One part of the problem is that 400,000+ RVs are sold in the US each year according to RVIA. Second is very few new CGs are being built. The only new one I've seen was at a casino. Many older CGs were built on land that has since become too valuable to use for a CG and wouldn't be viable today. How many new ones are being built near the ocean, at a lake or other desireable location? Is building a new CG financially feasible today?
We camped in California last August. One large CG was nearly empty, but that was in Bakersfield. The other was a Thousand Trails near Gilroy & Monterey which we booked waay in advance. Very hard to find a site there. It was so hot that everyone was inside - like a ghost town. I've been hearing for years that it is hard to get into CGs in Ca. It's only going to get worse over time.
We used to have a membership in a CG org. in Wa. Their flagship CG (near Can. border) was extremely busy in the summer due to them overselling memberships. If we ever tried to change dates less than 60 days out, we were SOL. We spent thousands of $$ on a membership only to find that getting a site there was almost impossible in the summer. We are now Thousand Trails members and can camp up and down the US west coast but still need to book as far in advance as poss. in the high season.