It's more complex than that. For one, there were many nations of people on the North American continent before European contact, each with distinct languages, cultures, traditions, etc. Then, people from many different nations immigrated to the United States, again, each bringing with them their own language, culture, traditions, etc. And then of course there were the African slaves, brought here against their will, once again, bringing with them their own cultures, etc, etc. Over time, each district culture, language, and tradition was eliminated and replaced by a singular, dominant cultural hegemony, that of English speaking protestants of almost exclusively northern European ancestry. That hegemonic order was maintained through force, repression, violence, and in some cases literal genocide.
That hegemony had been maintained for several generations, but it has been weakening over recent decades, as groups seek emancipation and autonomy. African Americans, indigenous Americans, Spanish speaking Americans who are recent immigrants or descendents of recent immigrants, all these groups (and more) are slowly eroding the dominant hegemony of English speaking people of European ancestry. As the hegemony erodes, distinct cultures will be able to emerge/reemerge, and/or many distinct groups of people will organically evolve along different paths, due to different geography and climate, economic conditions, history, etc.
It's true that the differences between us aren't nearly as significant as the differences between the various nations of Europe - YET - but that is because of the hegemony that has been in place, that had made the United States relatively culturally homogeneous. That will change. It already is changing. The hegemony is slowly (or maybe not so slowly) breaking down, and that will lead to ever increasing cultural and ethnic diversity. New, distinct nations of people will emerge, existing nations that had been violently repressed and forced to assimilate will reassert their autonomy, and this fiction about the United States being one nation of people will be exposed. It's already happening.
Ok, but even if the person living in Seattle and the person living in Lafayette are (at this moment in time) not as different as a person living in Zurich is to a person living in Lisbon, they might be different enough in their conception of what America is, or should be, that is ridiculous to act like they are each a part of a singular American consensus. The folly is not on all of us, because there is no "us."