Countries clearly have a large influence on culture

To influence is not the same as to dictate. And the ways that a country influences culture of its citizens are, most of the time, shitty - cue to the linked example of Vergonha. (It's actually a mild example, when you remember that massacres are a damn efficient way to have "a large influence on culture".)

And, sure, there are even milder versions of that. And considerably less efficient.

culture often forms around countries as people in a country share borders and law and politics and all that.

And it forms also across the borders too, to such an extent that "we have the same head of state" and "we're subjected to the same laws" become just a drop in an ocean. Food gets shared, people learn each other's languages (or make a contact language in the spot, that eventually is passed to their children), fashion and architectural trends get mimicked... even the laws get mimicked. Or they simply are born in a place and move 5km next door, and that happens to be the other country already.

Of course, as long as the countries aren't artificially trying to prevent that from happening.

Anyways, we can agree to disagree if you insist. I do think you’re being slightly closed-minded in this case though, but it’s not a big deal.

I don't think that I'm being closed-minded, but that I'm taking more things into account than you are.