Immigrants can be very successful in America, they have family and friends who want to come as a result. We're also much more of a melting pot than most the rest of the world despite what places like ERA want to pretend
Not sure if anywhere else in the world I can declare myself a nationality while looking deeply unlike the majority of everyone there other than america.
Tho lots of it is marketing, and some of it unfulfilled, that notion that anyone can be american is very unique I believe.
I think a lot of other say "1st world" countries that treat their citizens well are much more difficult to emigrate to... or much more difficult to attain status once you are there.
I think it's particularly hard for natives to recognize how difficult is it to actually emigrate.
The U.S even with all it's problem is I believe easier than any other 1st world country to emigrate to.