The reverse also works. People see a block of land that's "unused" so they simply move there and start "developing" it. Sometimes they'll legally own the land

The reverse also works. People see a block of land that's "unused" so they simply move there and start "developing" it. Sometimes they'll legally own the land ("adverse possession") but usually it doesn't matter. It's theirs because they're there and they're willing to defend themselves and "their" land even if they don't have a deed. So sure, go ahead and buy a bunch of land, in the USA or elsewhere; you'll need to be prepared to enforce your ownership stake or lose it.