But quite often, policies of one nation affect far more.
For example, it could be domestic policy of Iran to restrict travel through the Hormuz Strait. But the entire world depends on the Hormuz Strait. If China steals territorial waters from other nations like Vietnam, it is arguably between those nations. However, the South China Sea is important for every nation due to its importance in maritime commerce. Or when some nations decide to become tax havens, they effectively roam away tax income from places where the business actually takes place.
It’s too easy to say countries should stick to their own and not involve itself with other countries because in a globalized world, policies of other nations fundamentally affect other nations (of course depends a bit on the policy).
And then we haven’t even gotten into the matter of conflict. There is international law that depends on other nations maintaining it. If a particular nation (or entity within that nation but that nation is too weak itself) is committing genocide, you require international action to tackle that. It’s hard to argue that e. g. NATO should let genocide in Bosnia happen simply because it is an internal matter of Yugoslavia. Guess what? Yugoslavia was in a civil war and could not tackle that. Then you effectively say as NATO “we really can’t be bothered fo stop genocide” since the way they argue of tackling it is impossible.
In this modern age there is no longer a need for it. However historically I have been in favor of imperialism because it helped civilize and advance inferior cultures of people where cannibalism and human sacrifice were once commonplace, just as they were in Europe before Christ’s message of love for humanity spread.
Take people like Socrates. I mean such. I agree with you that existence of such people seems rather unlikely today, though they exist. The problem is they are not interested in anything like taking over and aren't qualified for that or trained for such things, they live normal, simple lives, work mostly unnoticeable jobs etc. But if some group os such described people comes up who manage to reach highest levels and still to remain who they were before, the possibility is still there.
That is not to say that sovereign nations should not have an interest in what is going on in the world that might end up affecting THEIR sovereignty and, eventually, their existence.
