Just like US states, most European borders are not drawn according to ethnic groups, and their names are similarly historical rather than descriptive. The nation state is a 19th century fantasy.
As for your belief that European borders are unrelated to ethnic/cultural distinctions, do you know much about Europe? [edit: blocked from replying] Were there truly no pre-19th century differences between the peoples and cultures in Finland, Norway, Sweden, and Russia? and no-pre 19th century differences between the languages and cultures of France, Spain, and Italy? [edit2] yes, being unable to quickly "reply" makes it difficult to converse in HN :-). [edit3] France is named after the Franks. Sweden (Sverige) after the Svea people. Finland/Suomi after the Finns. Russia after the Rus. Etc. Each of these has an ethnic, linguistic, and cultural core predating the 19th-century nation states.
But there’s certainly a “California” both in the USA and Mexico and it’s not confusing anyone.
I was born in Finland and have also lived in France.
(Edit: I see you replied to me by editing the parent post, but it makes the conversation impossible to follow if we both do that.)
Because one is called California and the other is Baja California.
In my mind Finland and France are examples of European countries that have pretended very hard at some point in their history to be single-ethnicity nation states counter to how the actual population sees themselves.
France is named after the Franks, but if you went back in time to 1700 and asked random people within the country's boundaries the question "What's your ethnic group?", the answers you'd get would include Breton, Basque, Catalan, Provencal, Alsatian, etc. — a minority of people within the Kingdom of France would identify primarily as "French" (and nobody would have heard of the Franks, a people that existed a millennium earlier).
If you asked that same question in Finland in 1700, you'd find a lot of people who identify as Swedes. And why shouldn't they? It was an eastern province of Sweden where every modestly educated person spoke Swedish. You'd also find people who identify as Sami, Karelian, etc.
Flash forward to newly independent Finland of 1918, and you'll find politicians who are adamant that it's a nation state. Yet descendants of those Swedish-speaking people are still there, and so are the Sami speakers, the Karelian speakers, and so on. Some of them will identify as Finns while others never do.
Maine is not called "North Massachusetts"