Too many liabilities, squatters, delinquents, etc for private landowners. It's a legal headache to create a "camping ground" on your property in the US.
True, though some states have 'recreational use statutes' that limit landowner liability for allowing public access to private land. But there are still many risks and headaches involved, even with those protections.
https://KOA.com/campgrounds
Sites like Hipcamp help people market their private campgrounds online. Lots of private places listed here.
https://www.hipcamp.com
On top of that, sometimes state parks/campgrounds can actually be just leased private land.
https://www.texastribune.org/2023/02/15/fairfield-lake-texas...