If we want to solve this at the society or community level, there needs to be more opportunities for low stakes interaction. Places that people can passively gather around a communal activity. I'm reminded of the ladies dancing together in public squares / parks in China. They're usually a group, but mostly anyone can join in. You can just follow along and interact as much as you'd like. If you want to leave, leave. If you want to stay and chat, stay and chat.
Downtown San Mateo for example has the potential for this. It's already a closed off street where people go. But today there aren't group activities there that encourages passive interaction, people are still in silos. Perhaps if there were some games / puzzles, chalk boards, townhall type of table setup, that'll encourage passive interaction.
OP gave the thread a very good and valid suggestion. Treating this as a societal problem - for "society" to solve - is lazy thinking.
If you want something you've never had, you have to do something you've never done.
My town does an annual party. I heard about it and showed up to volunteer. I did that for a few years. It wasn't as productive in producing friends (I'm in a different location than before that is more insular) but even so, it got me out of the house and, for the few months before the event, was pretty much fun.
These kinds of things are often available if you just look around. It doesn't require knowing people ahead of time and is low stakes. If nobody is friendly, it doesn't matter.