It's a little of both but the built environment is the primary issue.
We have neighbors - sometimes we need someone to grab a package, or we make too much food and we share, &c. or we run into each other walking to a restaurant or through the park. But this isn't the norm. We live in a neighborhood built before the introduction of cars, so homes are built a little bit closer together, but not too close, and we have mixed-use developments and a good level of density to support restaurants and other amenities.
You can't have spontaneous interactions like that easily in the United States because we build too much sprawl, visiting people or showing up to a bar requires a drive, and in the end you wind up just staying at home participating in surrogate activities like social media.
It really comes down solely to cars and car-only infrastructure that degrades our social interactions to an extreme extent.
-edit-
I do want to mention, at least when I was a kid/teenager I recall we used to show up to people's houses uninvited/unannounced too. But we did not talk to our neighbors. That was a weird thing. There are some cultural things here. But also even if we wanted to visit someone, well, gotta hop in the car. Maybe stop and get gas, and the next thing you know, eh it's too much effort. Might as way stay home. That's kind of how that works. The car-only model that is implemented in most of America, particularly the cities not so much rural areas, is a leading cause of cultural and social malaise I believe in the West.