Sounds like you're thinking inside of a fairly small bubble. If you picked 10 people, at random, from the 800k residents, I assure you that there would be substantial differences.
Off the top of my head, you might get SF State students, tech bros, Chinatown senior citizens who have never left an 8-block radius and don't speak english, Marina moms, Mission District multi-gen families. I mean, come on.
Maybe if you were only picking from people working at tech cos, but even then my experience does not match yours.
The point is that no one in practice selects a random sample of the people living near them. Everyone they meet is from some self-selected sub-group—the people who live close to X park, the people who work at Y place, the people who shop at Z store. And the larger the city, the more people there are nearby you who are like you, so your total variety experienced will be smaller unless you're actively going out of your way to go places that you don't normally enjoy.
So while OP may be wrong about a random sample of people in SF, they're probably correct about the people that they know in SF.
In a small town everyone shops at the same store, visits the same parks, works out at the same gym. There's only one library and a few restaurants, so there are fewer opportunities to self-select into smaller groups.
That's the point, isn't it? You need to try, and have both the determination and "skill", to find variety when the environment gives you homogeneity.
I'm sure I could find an elementary school teacher, or an insurance salesperson to chit-chat with in SF or NY. I never tried, cause why would I? These aren't Pokemon to collect. It just so happens that my gym in the middle of Manhattan filters quite strongly for a certain income level, and thus age, but also profession, etc. In my current gym we all just go to the same class together.
Next time I'm in SF or NY I guess I can invest the time and effort to join a facebook group to meet school teachers, if that's what we mean by skill here.
Off the top of my head, you might get SF State students, tech bros, Chinatown senior citizens who have never left an 8-block radius and don't speak english, Marina moms, Mission District multi-gen families. I mean, come on.
Maybe if you were only picking from people working at tech cos, but even then my experience does not match yours.