Big city seems to be the key. Loneliness is the default in a modern city, because we can. It used to be that daily needs led to friction that sometimes (if people were social enough) led to interaction (introverts were much less likely to engage so it usually took a lot of friction before interaction becomes possible). Schools (and universities) still provide the largest field for such interaction. Everything else has been largely automated away.

If we accept that this is a problem, then we should think about ways that force people to be near each other again, somewhat artificially. This is different than building a bar or making a meetup and expecting people to come - only the most extrovert want to go there.