I can't say whether it's unique to the US, because I've only lived in two other countries before moving here. Also, I've spent all these years since I moved to the US living in the same county of the same state, a state so notorious for how hard it is to make friends, that there's a name for the phenomenon: Seattle Freeze.

On the other hand, I've talked to a lot of other immigrants who lived in different states before this, and the general consensus (in that admittedly small sample) is that the US is definitely different from South America or Europe in that sense.

For context, I lived in Chile before I moved to the US, and Chile is the country in South America that tries the hardest to be like the US. Even in Chile, it's easier to have a richer social life than here, despite longer working hours and longer commutes, which both result in having much less free time. My own theory is because you get to socialize more at work and, if you have a kid that goes to kindergarten or school, with other parents. Here? "Not so much" would be an understatement.