Actually, many of them don't. I have a friend who is a dual US/UK citizen born in the UK who had never lived or worked in the states until she decided to get a

Actually, many of them don't. I have a friend who is a dual US/UK citizen born in the UK who had never lived or worked in the states until she decided to get a summer job there in 20s; she was informed of the requirement to file back tax returns when she submitted her W4. It makes absolutely no sense for the US to try to lay any claim to her earnings or expect her to file tax returns for any period of time before that.