I think the reason for this is actually pretty banal. Americans mostly don’t do deep, lifelong friendships. This is probably because they generally prefer wide social networks with lots of shallow relationships (nb I do not use ‘shallow’ here with any kind of value judgement, I’m being purely descriptive). Partly this is because if you’re ambitious you’ll have to move around and so it’s better not to invest in relationships with people too much because sooner or later one of you will probably have to move and a long distance friendship is not the same as one with frequent face to face contact. If you keep things lighter the pain is still there but it heals a lot faster. You want wider networks so that other shallow friendships can fill in the empty space. These types of friendships are easier to start because none of them take up too much time.
Adult immigrants are mostly not expecting to be able to make deep connections (they may be fleeing the very idea of deep connections) and so they’re happy enough to at least have American style shallow relationships and truly can feel that they’re being treated as equals.
In Europe, on the other hand, the general preference is for fewer but deeper and very longlasting friendships and many Europeans look down on shallow friendships as phony or manipulative (I think they’re neither). Immigrants have a much tougher time finding a social circle because the natives’ social circles are already plenty full thank you very much and they’re not into the idea of meeting new people for the sake of meeting new people.
There are probably ways around this hurdle and they probably differ by country. The only example I’m pretty sure of is Poland, where there definitely is, but that’s a topic for another post.