It depends on the park and the activity. For example, Great Basin National Park has no entrance fee, but does have fees for a cave tour, campsites, and RV dumps.

In my experience, the vast majority of public land (forests and BLM land) is free to enter and recreate on if you aren’t using services like a campsite. National parks are more likely to charge entrance or parking fees but you are also more likely to get things like trash service and paved roads.

That being said, it’s still pretty cheap. You can get an annual pass for $80 that covers you and everyone in your car.

BLM land is often entirely free, but doesn’t have “campsites” as many would expect. State and national parks often have campsites with running water, flush toilets, even hot showers, and quite in the middle of nowhere.

BLM campsites are often a flat piece of ground that looks identical to the last seven thousand square miles.

Sometimes they will have a tiny retaining wall/perimeter to make it flat.

Given that no one is paying for your transportation to or lodging in the park, it's expensive in any case regardless of a typically modest admission fee. That's almost surely true in most places.

Admission localizes the expense to people who attend. If it were "free" to visitors they'd still be paying.

The big national parks are basically amusement parks at this point. There are still plenty of free areas to go with less amenities, and many national parks are still free.

You also need to pay for clothes and food. The land of the free is very expensive.