You're arguing that the actions of individuals have no impact on the collective actions of humanity, the sum of 8 billion individuals.
Similarly, you probably never had a conversation where someone said "Gee, I'll stop burning fossil fuels now!" But when you use public transportation, or patronize businesses on foot, policy makers are motivated to continue supporting such options. Not for you personally, but for you and others like you.
I'll add dancing to this list. It's certainly possible to just show up, do the steps, and leave, but it's an atmosphere where talking to people is very easy.
If you go to a class for a few weeks/months, you'll start hearing about other classes, or events. You also might end up finding that you have other connections with some people who you dance with.
I speak from experience here. I'm not very good at people, started dancing for that reason, and my time dancing has helped noticably, according to an old, long distance friend.