BRD and DDR were in a quite particular competition. The DDR contained less than 20% of the German people, and West Germany would happily have absorbed them all.

The capitalistic world state system was only completed in the last 50 or so years and was of course seconded by the Stalin machine. Its purpose is in the first instance to keep working populations in place, which is why all the states agree to it as they form. The system holds solid where capital moves (as did 'communism' and Comecon) but the peoples don't.

It is only when you think of individual states and not the system that you can fail to notice that we get the same result if they all try to keep their citizens in, and if they all try to keep the others' citizens out.