I've found it to be very obviously the case that the main reason this happens to me is that everyone is moving around all the time. People scatter to all corners of the country (or even the world) and time zones and travel times just make it impossible to meet and maintain friendships. None of my friends growing up live in the same state as me.

I don't have the link, but this claim almost always gets made in these kinds of comment threads, and then someone points out that American mobility is in fact lower today than it was in the late twentieth century. And different countries are different but still report growing loneliness, etc.