Can anyone elaborate on this? Speaking as one of them, I'd hazard to say that the majority of 'white people' in the United States are so far distant from their 'ethnic heritage' that we'd just define it as 'American'.
Mexican-Americans celebrate their Mexican heritage, Japanese-Americans celebrate their Japanese heritage, etc. What does your average 'white person' celebrate? We don't know a heck of a lot about my mom's side other than that her father is French Canadian and her mother is Polish American. Dad's side has been here since the 17th century (according to my grandmother anyways).