If you read between the lines of what Terence Tao is saying - which, by the way, the most charitable summary is, "Isn't Dunbar's number interesting?" - he is really saying, "It is hard to make friends as an adult." Extra so if your day to day is something esoteric like academic theoretical math (read between the lines: really boring to most people), and if you are right leaning or libertarian (read between the lines: unfriendly as a matter of policy).
It's extremely expensive to get a physical space these days, there are no old warehouses or cheap community centers where you can just run a community event. They cost thousands of dollars to rent for a day, so anything short of a ticketed event or selling lots of alcohol is not viable.
The places that do still have a physical location are often almost grandfathered in where the organisation bought the land decades ago and now pulls in enough from investments to continue perpetually, but could not afford to buy or rent the space at market rate today.
