Of course, you know this. It's why people think big-city folk are rude. No one smiles at each other. No one says hi. But I'd offer that this behavior is less about rudeness and more of just a numbers game. There are simply too many people to say hi to. Every morning, I ride a train with hundreds of people I don't know and usually don't recognize. Then I walk through a major train station surrounded by thousands of bustling strangers. Then it's into a subway taking me underground to work. And even though I'm going to the same place every day, varied only by which train I catch, I almost never recognize the strangers, because there are so many. Also, everyone in the city wears masks 24/7. Did I mention that?
Here's me on my way to my office job.
There are two points here. First, when you live in a small town, you don't have a million strangers to say hello to during the course of the day. It's a manageable number. But, more importantly, you have an extra incentive to say hi: You'll see them again. You know them. They're your neighbors and a part of your town, even if you're not on a first-name basis. Pretending they're some anonymous stranger would be the weird thing to do. But when every person crossing your path is about to float off anonymously like the remnants of Robin Thicke's career, it's really only the weirdos that stop and say, "Hi there!"