Why do people put so much emphasis on the country you were born in?

I was born when my parents were studying abroad in the United Kingdom, and since my parents were not citizens over there I did not get British citizenship myself. I am not ethnically British (in fact I'm ethnically Chinese) and I've lived in the US since I was two years old and only have a US citizenship. Yet people always talk about how your identity=the country you were born in, when I'm about as British as a taco is.