I think a key difference is that when people are in their home city, even if they're newcomers, they have at least some fixed outlets for meeting people through work, organizations, exercise, etc.

People who are away from their homes feel a bond with other people away from their homes--especially if they share a language in a place using a different language--and it doesn't just apply to nightlife. In addition, they have an urge to talk about their experiences, and it's hard to do that if you're struggling for basic words, so you may seek out other expats or travelers when you're new to a place, then develop friendships with locals as well as foreigners if you're in a place for awhile.

Surprise, even some of us old farts manage to continue new friendships using social media.

In small towns in the U.S. South when I was a girl, people nodded or waved or spoke even to strangers, and the Guatemala people here are the same. In all the Latin American countries I've been in, strangers will wish me a good appetite as they pass my restaurant table. Strangers don't generally do that in the U.S. The degree to which one talks to strangers and the amount of personal space one expects are both shaped by culture.