I think churches have often fulfilled this role historically; but now that job is served largely by colleges (especially those with a high-cachet brand-name). They're a modern 4-year debutante ball: the dating pool is filtered both by age and socioeconomic status, and most students are packed into tight living quarters, maximizing random social interactions and leading to emergent trust/reputation networks. There's a reason that "we met in college" is such a common answer from married couples.
Bryan Caplan and Robin Hanson make sharp critiques against secondary-education institutions, arguing that signaling of grit and conformity is primary, while learning is secondary; but IMO the dating-pool petri-dish aspect of college (often subsidized by parents starting to think about grandchildren) goes under-examined.
Where I grew up, the Catholic Church (the only religion in town) didn’t really play the community-building role the Americans tend to ascribe to their churches, so I too find it baffling myself. However, I can’t help but notice that both my father and my best friend have met their wives on religious pilgrimage to holy site.
There _are_ social outlets in the church and sometimes ancillary to the church (e.g. Knights of Columbus) depending on how social your church is and how hard you're looking for social opportunities. However, I also get that Catholics can get weird about dating (at least anything on the premarital side).
And yet, there's distinct differences in around pre 25yo and post 25yo on average, to the point where childhood development psychologists use that as the current baseline.
