I don't think that applies anymore these days. I am from Poland and culturally I have more in common with most Germans than with many of my compatriots.

And my point is that the idea of "their piece of earth" is outdated. Apart from silliness and unnecessary politicians it leads to real problems, like countries deciding that they will burn oil and coal on "their piece of earth" and not care about climate catastrophe at all.

I am Polish too and I’d say were are on average very different from the Germans. A common, Polish-German state would be very hard to govern to due constant conflicts in values.

A lot of Germans feel the same about the Bavarian-German state.

Then what about Switzerland who speak 3 langues (4 officially). We talk English together and the French part is generally more focus on France than Switzerland (I'm from that part)

I guess you are still very distinct from say Belarusians or Japanese? In a world with no borders, the Swiss, Belarusians and Japanese would need to somehow agree on a common set of values that would govern this borderless world. And it’s hard enough to agree on policies in a single country.

We already have a lot of nearly-worldwide treaties, so there is something like a common baseline already.

I'd say it's enough to look at the US as an anti-example of how politics look like when you create a giant political body which governs people of radically different value systems. IMO, the solution is to pass more power down, to local bodies, not up, to continental or global behemoths.

The problem is that more and more issues are global. No single country can fix the effects of climate change on their territory, you need to cooperate on a global scale to do that.

Same e.g. for combatting tax evasion. Many other areas do not strictly require transnational collaboration, but it's immensely helpful. For example, the power network in Europe is incredibly resilient because it is shared across dozens of countries. So every individual failure is rather small, even if it takes a large nuclear power plant offline.

There's some funny business going on in the phrase "their piece of land". It's not like each citizen has the same ownership rights of every bit of the country. If I own a bit of land in Japan, why does it suddenly become "their land" for the purpose of determining laws?

It has hardly ever been that clean cut in practice. And the attempts to enforce the strict divisions have not been pretty.