The difference is that - excepting about 1.4% of the population - everyone here in the US is either an immigrant or descended from immigrants. Most of them long after the Mayflower sailed. However long it takes to create a new capital-E Ethnicity, it hasn't been long enough.
German citizens didn’t evolve in Germany. Any attempt to delineate ethnicity based on how long you ancestors have been in a country is just a veiled attempt to argue that you belong and they don’t.
And that is what binds them together, they all took the leap of faith, they all seperated from the old world, they all brought only their work and their spirit. The us is a phyle of choice and you must have made that choice to belong to it. This Choice is also the freedom so often referenced. Which also means you can leave the us by abandoning its values.
> excepting about 1.4% of the population - everyone here in the US is either an immigrant or descended from immigrants
Your data and percentage, is very wrong. America has significant Black and Indigenous (usually referred to as Indian or Native) populations. Around 15% Black and 3% Indigenous. Combined, they make up around 18% of the US population, with wild and vigorous arguments they are even a greater percentage than that (20% or so).
Who cares if most people in the US had ancestors that came from somewhere else? My English ancestors have precisely no bearing on the way I live my life any more than my German, Dutch or Polish (well, they came from what is now Poland, but would never have thought of themselves as polish). The child of immigrants in Germany is going to be far more German than I am despite my ancestry.
American culture is undeniably real. American values and beliefs likewise.
Is the only thing that decides an ethnicity how far back your ancestors have been procreating within a country’s current borders?
Culture and values is a better delineator, and it is pretty undeniable that America has a distinct culture and value set.