A border is a community saying "we believe certain things and within this physical territory, our beliefs prevail, by force if necessary". And if someone says "

A border is a community saying "we believe certain things and within this physical territory, our beliefs prevail, by force if necessary". And if someone says "I believe those things too, can I come in?" that's OK. But if someone doesn't believe those things it's neither clear why they would want to come in, nor why that community would want them to, since whatever reason clearly isn't for the benefit of existing community members. The beliefs are written down as the law but they aren’t set in stone - a candidate who ran for office on a manifesto of an open border and won sufficient votes could rewrite the law. I’m not sure if there is an electorate anywhere in the world that would go for it.