In Austria, when you want to go on a hike, you just go. Sure, there are some popular routes that are crowded, but 99% of our national parks are completely empty. As soon as you can't drive somewhere and it takes a few hours to hike there, it's pretty empty anyway.
Are there so many more hikers in the US, or is this about limiting access to the most popular routes?
Decades past, nobody but the most adventurous would visit these places and reservations were never an issue. Then, with increased visibility/awareness (starting from things like the American Automobile Association magazines, to TripAdvisor, to these days TikTok and Alltrails) visitation kept going up and up and up and up and the lands and agencies couldn't keep up.
This is especially the case with our major parks (Yellowstone, Yosemite, Zion) but even smaller places can become an issue with enough social media exposure:
Horseshoe Bend in Arizona: https://abcnews.go.com/Lifestyle/instagram-tourism-booms-hor...
Or Muir Woods just outside of San Francisco: https://www.sfgate.com/bayarea/article/Wildly-popular-Muir-W...
So places have to try different strategies, including but not limited to lotteries, park-and-ride shuttles, timed reservations, rotating closures, etc. to both enable access but also protect the very resources that people are flocking to see.
It's not just a top-down effort either. There are grassroots NGOs dedicated to trying to balance visitation vs preservation, like Leave No Trace (https://lnt.org/) and the Access Fund (https://www.accessfund.org/).
It's a hard problem all around... you want people to be able to easily enjoy the lands that their tax dollars pay for, but when there are TOO many people (and tourists, both domestic and international)... it gets crazy.
EDIT: I've always wondered why more private companies don't create shared buses to help backpackers hit the entrance to the park. A lot of the times just getting to the entrance is what makes folks use their cars, and if you could park in a nearby town and take a shuttle in and start your backpacking that would go a long way. I know some parks run shuttles inside the park but I'm talking about a more backpacker/cyclist specific transport just to the entrance to minimize impact on the park itself.
Don't they, though? I've seen these at Zion, the Lost Coast, the Cascades, Olympic National Park, etc. And around big cities, often the city/county itself will provide a direct-to-trailhead bus.
No doubt this is sometimes true, but all the permits & reservations I'm aware of in parks are for the hikes themselves, it doesn't matter how you arrived there.
Some, like the most prominent parts of National Parks, are specifically managed to enable easy access (and also ADA accessibility, like wheelchair-friendly paths). There are also preservation-focused areas, especially but not only Wilderness (our highest form of protection, designated by Congress, prohibits motorized traffic and most buildings, often no use of machines like chainsaws or ATVs, though most will allow pack mules and such). Many multi-use areas (National Forests, BLM land, etc.) also have minimal facilities, often just a dirt lot and maybe a pit toilet if you're lucky. On the other hand, some smaller National Monuments might have no accessible areas EXCEPT for a visitor center and parking lot, usually to protect some extremely limited or fragile resource.
The USA is HUGE and many lands are public and/or protected for various reasons, to different goals. Not every trail is a National Park, and not every National Park is there for hiking or backpacking.
And it's also not merely a matter of land designation. Crowding can happen even in designated wilderness areas, often the result of a place going viral or being in close proximity to a major city or whatever.
Despite being a highway rest area it is illegal to stop there from Memorial Day to Labor Day without a pre-purchased permit. I generally avoid the area during high season so I have no idea how strictly this is enforced should you ever have an urgent need for a toilet.
These days though, even day hiking is increasingly permitted :( I live near the Oregon Cascades, an area that used to be unpermitted but got too popular. A few years ago they introduced a permit system (for both day hikes and overnights), but implemented it terribly... a large % of the permits were released on a single day and were sold out within hours.
People ended up hoarding multiple permits for multiple weekends because they couldn't be sure which weekend they could go (or what the weather would be like), then 40% of the permit holders never showed up on their given reservations... so nobody got to go. They've since tweaked the system.
https://www.opb.org/article/2022/02/16/wilderness-permit-cen...
Your point absolutely stands -- that day hike permits are still largely unusual across the entirety of the US public lands systems -- but more and more places are getting so popular that they do require reservations. It's one of the downsides of social systems like TikTok or Alltrails, which improve discoverability but also focus public attention on the few best trails rather than the thousands of "still pretty good" trails.
For one thing it generates a lot of profits for both recreation.gov and (somewhat less) the parks. That's unavoidably going to be a motivation for ever more limits and lotteries.
For me that's really soured the idea of going to these places. In the 90s I'd go to any park, show up and hike wherever without any limits, it was great. Now it's hard to be able to ever go, let alone luck into the limited slots.