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.