Modern infrastructure (physical and social) can't handle massive population migrations.
Consider a basic US example. City's deal with their homeless populations in different ways, but if any city is overly generous it quickly gets swamped. Further, cities can export their homeless by being overly draconian. Net result, it’s basically impossible to deal with the homeless problem at the local level.
Now, scale that up to 2 Billion poor people around the world.
PS: The tragedy of the commons applies to far more than just land. Closed national borders allow for significant social progress which is otherwise impossible as it would otherwise need to scale beyond what any country could possibly support.
Consider a basic US example. City's deal with their homeless populations in different ways, but if any city is overly generous it quickly gets swamped. Further, cities can export their homeless by being overly draconian. Net result, it’s basically impossible to deal with the homeless problem at the local level.
Now, scale that up to 2 Billion poor people around the world.
PS: The tragedy of the commons applies to far more than just land. Closed national borders allow for significant social progress which is otherwise impossible as it would otherwise need to scale beyond what any country could possibly support.