It costs money to maintain public land, but that's not any less true in the other countries I'm talking about. The difference is that that cost is more likely to come from national budgets, rather than from fees, which is better in a lot of ways: it's more transparent, and is a progressive rather than regressive tax, is better for tourism, etc.
Also note that a lot of the non-fenced land in i.e. Norway isn't public land, it's private land, and it's still accessible to the public due to cultural norms. This may seem weird that there's a social expectation that people will open their private property to public use, but the flip side is that there's little cultural opposition to government helping maintain property. Many hiking shelters, for example, are on private property, but built and maintained by the government.
It costs money to maintain public lands like national parks. The neanderthals who don't understand Leave No Trace principles cause massive amounts of damage that needs to be prevented, policed, and repaired. Chores like trail maintenance, habitat restoration, wildlife studies, trash removal, toilet upkeep, etc. etc, all need to be done to keep the parks as close as possible to wild, so that the next generation can enjoy it. Entry fees fund the conservation of these public lands.