Because anyone can become American, and American isn't an ethnicity, it's a nationality. There's a particular... I'm not sure how you'd word it... (exoticism?) in exclusivity. A German can be proud of his German ethnicity, and same goes for the French, Swedish, Romanian, etc. You can't just become an ethnic German, you're either born like that or you aren't.

But (white) Americans don't have that, they're a % of each with no proper identity with any. America's culture is the melting pot, the whole point of which is multicultural. But immigrants and their kids from other countries have their own ethnicity, they can be a proud Indian American, Latino American, Chinese American, etc. These people don't need that DNA testing crap because they already have an identity, but they're also American, equally so as any other.

This is why Americans focus on race a lot more than other countries. It's also this lack of cultural identity that gives rise to "white pride/supremacy", creating a "white culture" to be proud of (AFAIK, this isn't prevalent in most European countries?). It's literally the only "exclusive" thing left to identify themselves with.