Political Memes

Sure, effectively all land is claimed by some entity, but not to deprive people from being able to make use of it. The US for example needs to claim ownership of its territory to have it recognized by other nations and enforce its own laws. Otherwise, someone could lure you into the wilderness and kill you without penalty like it's Runescape. And even "owned" land will be subject to emminent domain when the needs of the many demand it.

But buying undeveloped land for homesteading is cheap; you only have to have a token price for depriving the public of its potential value by your reservation. Otherwise, nothing prevents someone from taking it all for themselves for free (which really would leave nothing for others) just to not use it. Even if you did it illegitimately and just started using fresh land without paperwork or anything, you would likely still have recognized rights of ownership through common law squatters rights just by using it effectively for some time. But if you wanted to say, vote, or get mail, or have utilities, or have road access, or otherwise engage with larger society, the government would likely at least want property taxes. After all, getting that to you would take from the pool of resources used for the common good, and you need to contribute a fair share.

If you really wanted to forgo the social contract entirely, nothing is really stopping you from going into deep wilderness 100 miles away from civilization and fending for yourself, but people recognize that the benefits of being a member of society greatly outweigh the costs. Other animals do have to work to live and reserve their own territory. They just don't use anything as formal as currency for exchanging work for resources, and reap fewer rewards from less specialization.

I personally support UBI but trying to pretend nature is somehow more fair than modern human civilization is just arguing in bad faith. The systems we enjoy are certainly flawed but also undeniably an asset at recognizing the rights of others to live. Nature's resource distribution system is literally a combination of luck and might makes right.