Back in the day, probably no country ever was as good at incorporating immigrants and making them feel welcome (or at least not un-welcome) as the US (though Canada or Australia might be contenders and Brazil and Argentina had their moments). Most Americans by birth are generally comfortable with immigrants (or soon become so) and there are informal protocols for helping newer arrivals adapt.
This was reinforced in the media, especially TV where immigrants as upstanding figures or first generation Americans with immigrant parents or immigrants who want to assimilate are all established figures with established tropes.
One downside of this is that Americans tend to view temporary visitors to the US as immigrants and interact with them in largely the same way. I knew a student who worked as a waitress at a US country club over the summer*. Toward the end of her stay the managers told her she “didn’t have to” return to Poland they’d be happy to sponsor her as an immigrant. She thanked them but said she didn’t want to stay in the US and relations got very frosty….
Another downside of all of this is that Americans tend to view all non-Americans as potential immigrants and many see interactions with them at least in part as preparing them for life in the US.
The TV trope of immigrants who are eager to assimilate (from Latka in Taxi, through Balki in Perfect Strangers to Raj in Big Bang Theory) convinces many Americans that many or most non-Americans aspire to American values. The modern age is inundated by popular media and people absorb attitudes and ideas about the world from media far more effectively than from traditional sources.
This is one reason that American foreign policy tends to be so misguided as it often operates under the assumption that the rest of the world is hungering for American values and they tend to force interpretations of what goes on in other countries through this very unhelpful sieve.
*there are a number of companies that facilitate working summer holiday trips to the US