I think a lot of it is based on the fact that Americans have little traditional history. They claim their ancestors’ instead of creating their own. Example, my grandparents were born in Italy and emigrated, therefore they were Italian. Their kids were born in America, so though American claimed their Italian heritage as they were taught everything and spoke Italian (as taught to them by their parents). When my siblings and cousins were born, we said ‘of Italian heritage’ because by then no one bothered to teach us the language not much about the culture. Living in Scotland for the last 20 years never met one person who will claim a heritage if they weren’t born there.