[gumby]

The US doesn’t have the highest proportion of immigrants, nor is it the most welcoming to most (I am myself an immigrant to the US, and was welcomed).

It’s in the top quartile of both criteria, though.

[pessimizer]

The US had the advantage of mostly eliminating the indigenous population. The only Americans who aren't immigrants are natives and the descendants of slaves. Other countries have large native populations.

[slaw]

Natives are also immigrants.