Your discussion of proximity hit the nail on the head.

In US culture, people are raised to expect to move around quite regularly. And you're right, once you're no longer near someone geographically, the friendship tends to fade.

The US misses out on some great cultural phenomena because of this. I've met people who have lived in the same city with their friends for 50 years. It makes a difference.

In 50 years, you can't hide your problems all the time, and you know you have people you can count on. It's a real blessing.