Where to even begin with this idea? First of all, many local governments in the U.S. already have public databases of who owns what land. This is a problem that can be solved with a SQLite database, it doesn't need a blockchain.
Second, controlling access to individual pieces of land through ownership is a fundamental function of capitalist states. It's even more important than coining money. Why would a government delegate that to the UN? What would the benefit be? If a state can't effectively control access to land, that's an existential threat and not something the UN's "brand" can solve for you. The potential downsides are huge for everyone involved - sorting out disputed ownership or inheritance now requires a call to Brussels or Luxembourg?
If this is supposed to be a reason why NFTs are useful rather than stupid, I still feel like I'm not getting it. That isn't to say there couldn't be valuable use cases out there.