Yea they were mostly rebuilding the physical world and not having to completely rebuild society from the ground. Small countries like the Nordics aren’t great examples because they’re so small and a massive part of their wealth came from selling oil to the wider world. The amount of oil revenue generated per person in those countries allowed them develop such a high standard of living along with really smart and egalitarian government policy and investment. That’s not really repeatable for a country as large as the US although money could certainly be used better.

France invited large amounts of immigrants from Africa to help rebuild so it’s not like they didn’t see immigration.

I think there are two main reasons that the poorest parts of the population are stagnating. One is poverty traps, and especially multi generational poverty traps are very hard to escape. Another is geographic location. Americans dont move as much as they used to, all around the world poorer people move to areas with more jobs and opportunity but today mostly richer people move and poorer people stay put in areas that are impoverished.

Immigrants largely don’t come with a set geography or generational poverty weighing them down which is why their kids do so well.