It's even more strange when I see 3rd or 4th generation children from immigrants call themselves "Greek" or "Italian" and many times they've never even stepped inside those countries nor speak the language

I mean you've basically hit the nail on the head except you're misunderstanding one important thing. They aren't 'trying to find one' they have one. Their culture IS that Irish or Cuban heritage and it wasn't retconned from 23andme or ancestry.com - it comes from the story they were told about their identity by their parents from an early age.

I worked with a French guy in Amsterdam. His parents were Portuguese, but he was born and raised in France. As far as he was concerned, he was French.

As I understand it, that's a French thing specifically, not just a non-USian thing. Like, if you're a citizen of France, you're expected to be French and assimilate into that culture, no matter whether you're a native Parisian, you moved there from Algeria in the '60s, or you're from some random other place and got citizenship via the French Foreign Legion. It's a specific sort of national ideology that's different from the American "melting pot" one.

It's actually kjnd of the opposite: America has the dominant culture going back generations. It's just that culture is very materialistic, so people try to find something deeper. That's my theory anyway. Besides, most of us are immigrants and I think a lot of Americans want some connection to their place of origin.

My own theory is racism. Other countries in the Americas are not obsessed with ancestry. But bigotry against Scots, Irish, Italians, Africans, Chinese, Polish, etc. ran / run rampant.

Jeez, are there people the English didn’t hate? I wonder if the overall disdain for other people the English had in the 1800s wasn’t what was carried over to the new world and festered into this.

It's also very much part of the 'murican narcissism culture, everyone has to be special in some way, no matter how shallow, made up or objectively irrelevant that is. I've known a few Americans IRL (I'm Swedish) at different periods of my life and no one else has ever come close to the level of mental gymnastics they do to feel special, cool, different etc. This really mirrors a lot of other things about the US, the classic image of early American towns with houses that have decorated facades but that's all it is, paper-thin lies to mask both nothingness and shittyness. And man do they hate it when you try to push your finger through those shallow shields they build for themselves.