Why ? People live in big cities, don't have to worry about other people around them, and it's still a community.
There is an assumption there (that isn't quite fair). Not everyone lives in a big city. I live in a remote rural area. I see (visually see) my neighbors maybe once a week. While it is quiet out here, and I can avoid stress, there isn't much to do. The nearest town is 6 miles away (~1K population), and the next larger is 12 miles. Like it or not, you end up doing a considerable bit of driving (in the car) just to meet basic needs. The nearest full service hospital is almost 40 miles. A long ride if you have an emergency.
The social scene here is divided into 3 basic groups: those who attend church (multiple times per week), those who attend (almost daily) services at their favorite bar, and those who have kids (and the typical events that kids are organized around).
The community, such as it is, is considerably more sparse than it would be in a large city. It is very easy to feel lonely, and depressed. Without the internet, my life would be quite different.
A second reason is the night sky. There is no way to properly describe walking outside on a dark cold night, and looking up. Thousands of stars. The occasional bit of debris reentering in a blaze of fire across the sky. Staring at aircraft lights, wondering if they are passenger, military or something else. The dark night sky isn't really dark at all, it is full of wonder.
You're describing a very small social network. If every person's immediate family and close social network is a "community", then "community" loses all meaning.
Do I care about the other people just in my apartment building? Not above a minimal base level of caring for anyone, apart from a few cute little kids. Do I know anyone else in the building? No, not really, not even the cute little kids. Would I help one of them in an emergency? Sure.
But we are a community in the sense that we have shared interests: that the elevators be kept running, that power be kept on, that the water supply be kept going, that the parking lot be plowed, that the garbage be collected. We may never have cause to work together in support of these shared interests, but the potential exists.
Other relations between members are possible as well. Students and faculty at a university, for instance. Or staff and patients at a hospital. Or a regional technology interest group. In these cases, membership in the communities is more tenuous, and is maintained by group participation, enrollment in the university, participation in a hospital treatment program, etc.
Virtual communities such as HN are almost entirely reliant on interaction for establishing membership.