Americans use ethnicity as a synonym for race. That's not formally correct. Wikipedia has it thusly:
An ethnicity or ethnic group is a group of people who identify with each other on the basis of perceived shared attributes that distinguish them from other groups.
If you don't know your ethnicity, then you don't have one. Except you do have one. White American certainly fits the definition.
Americans when they find out they're 1/823th of a footballfield finnish:
"OMG I AM SO DIVERSE! AND NOW A CERTIFIED MINORITY"
Americans when they try to do finnish things:
"Yeah no this is fake, its not possible for anyone to survive in 230f"
Makes sense though because in America, everyone is American regardless of background, race, etc etc etc so people, in their search for ways to differentiate themselves from others, latch on to their heritage.
Ironically America has incredible diversity within its borders. The average west coaster, northeasterner, Bible belter, and Midwesterner are completely foreign to each other. Plus a religious obsession with sports is another way we tribalize.
My brother got a DNA test and found we're mostly descended from Nordic people and not Germanic/Irish as we were told as kids. So naturally, I am now a Vikings fan.
I don't know, one guy has a real problem that needs to be addressed. The other guy is mad because people got mad at him for cosplaying an ethnic identity.
I still remember when someone came to /r/Wicked_Edge, a subreddit about straight razors, and asked to compare two disposable brands. People were as kind as possible.
I have Italian ancestry and I've always found these guys to be cringe, but I also get why they do it . Many people in the United States yearn for meaning and interpersonal connection in their lives. "Being an Italian" provides a prepackaged, very commercialized possibility of community with little effort required - you're just born to it, so instant acceptance, right?
The reality is often less Soprano's chic and more "nonno and nonnina were illiterate farmhands who moved to the US for a better life. Nonno died from mystery cancer and all of nonnina's bones dissolved after birthing her 15th child at 24. Now chew nonnina's birthday cake for her".
We need to do a better job of teaching people to approach community groups with respect and as themselves. I'm a severe introvert so it took me way too long to figure that out but it's really that simple. There are no shortcuts, be nice to people, ask respectful questions, befriend them, and suddenly you're invited to the party/dinner.
In no particular order, I have French, German, Dutch, Scottish, Irish, and a teensy tiny bit of "my great great great great grandmother was native American and we actually have the proof but nobody could ever tell without a DNA test so it only gets brought up when talking about obscure family genetic lineage"
Maybe it's because my family is super midwest-usa-bible-belt, and I never even found out about most of it until a genetics test when I got married to my now wife (we wanted to know if kids would even be a medical possibility with our various issues), but I don't identify with any of the places my ancestors lived in, so there isn't a particular culture I'd like to be part of. And to be perfectly frank I'm not sure I want to be part of any culture, I just want to tend to my forest with fair Goldberry my wife.
You do make a good point though, if you're looking to be part of something or feel particularly drawn to a culture after being immersed in what you think it's really like, I could absolutely see this happening with 100% sincerity.
It's lonely not being Italian American in NJ. I was psyched when Korean and Japanese kids started moving into town in the 1980s because suddenly it wasn't me and 25 kids who all shared a common culture.