This is a bit like saying we are not running out of water because there is plenty of water in the ocean. All that space in the middle of nowhere isn’t very useful because people do not want to live 50km away from cities.
It's extremely useful. It's just not in demand. Which is exactly what I just wrote. Land is not at a premium. Space in the cities is, like every nation around the world.
Obviously when people say land is scarce, they mean that the useful land that people actually want to live on is scarce.
Having a billion square km of sand in the desert isn't useful when someone wants to buy a place to live and exist in a society.