This topic interests me, and I'd like to understand your point of view on it.
You've said the US and Canada are mostly countries of immigrants, which is obviously true, but wouldnt this also be true for other countries?
The US and Canada are both "New" nations, though they've had numerous indigenous tribes for tens of thousands of years, themselves migrating from another continent over a long ago melted ice bridge. My point being that even though timespans vary, inevitably everyone came from somewhere else going back to our species first leaving Africa.
1.The african origin is hypothetic only, but that's not really relevant here.
2.We have to talk about very different time spans here than those of the genesis of humanity and such, Obviously none of the todays cultures existed back then. But if we're talking about time spabs relevant for eurasian ethnic groups, cultures, peoples and nations, we are talking about thousands of years still. The gap to the new american nations is obvious. The whole history of the US is no even 300 years. To put it into perspective St.Petersburg in Russia is considered a rather young city. It's only 315 years old. That is not to say that one is better or preferable in whatever way than the other. It is only to depict how deep some cultural roots go. And those roots do matter. And they do work in people's heads more than people realize. The cultures in the US ad Canada are still more or less european influenced in their core for example.
On an even smaller scale of maybe that span of the American history then there is that culture then int NA that everybody becomes an American or Canadian by the place of birth and/or citizenship. Couldn't work otherwise.
3.The other fact is the continuing immigration in those countries. That creates that special caseand a special tradition.
4.It is different in other parts of the world. For example on the huge territory that was the Russian Empire there was never atradition of the "melting pot". Nobody ever tried to assimilate smaller groups.It is also supported by the fact that those ethnic groups mostly continue to live where they used to live for thousands of years in some cases. they haven't left their lands to live in a different country/culture. They keep their language and culture, they live on as anethnic community to some extent.
That does create a different tradition, where you cultural roots are what matters, not your place of birth. In that tradition declaring yourself a Finn while your parents are Russians is strange.
5.Some modern european countries(partially influenced by the US btw. That's how complictaed those influences are. First Europeans basically created the new Americans and now american culture influences Europeans) do that shift nowadays and tend do value passport over ethnicity or cultural heritage. They often too have a hard time to understand my stance that legal affiliation and cultural heritage can be completely different issues. As I spent a lot of time abroad I was asked on numerous occasions if I would "feel" like I am russian still. They never understood how ridiculous that question is to me. I am want I am. I can't become anything but russian. Not if a housand pieces of paper say otherwise. It's not a matter of where I live or what language I speak or who my friends are. At the same time my heritage is as diverse as that of nearly anyone in my country, not unlike o that of modern day Americans. That btw makes Americans and Russians way more similar than most people realize.
Yes, in thusands of years our ancestors all did their share of travel
, but in out understanding it's the culture that defines you more and that you take with you to Mars if you go there.