Sometimes it's good to talk about serious things with someone not directly involved with your life (i.e. no skin in the game). People even pay for this as a service, I think it's called... um... therapy.
A friend of mine "admitted" to seeing a "talky doctor" because "there aren't any insightful bar tenders any more". This in the US.
I worked a the closing shift at a very popular bar in the West village, NYC around 2007-2009. I had my regulars, the work was simple. I'd get to know them and learn about their lives. I made a bunch of friends. People I would have never met otherwise, because of social strata or just having completely different lives that wouldn't intersect otherwise.
I'm not a huge fan of smalltalk strangers, the kind that you might have with a stranger on the subway or someone at the dog park. But for whatever the reason, those weren't the kinds of conversations that were occurring late nights at the bar :)
I've seen an engineer who had sat through a day of meetings on a topic holding his tongue until they went out for drinks after work (like most days). The moment beer touched his lips he broke into an angry tirade as to how terrible the decision was, and because we were "drinking" it was not considered out of bounds or reasonably punishable insubordination. Of course sexual harassment can have similar in/out of bounds elements so it's not like strict rules, boundaries, and exceptions solve all problems.
I don’t mean this at all as an inflammatory hot take, but drinking with people builds bonds in a very effective way which is very hard to match inside the office environment. I acknowledge though that there are obviously big problems with drinking as well and not everyone should do it.
The claim that people wouldn't feel excluded and disfavored for not attending but frequent social bonding over drinks because its optional is impossible for you to objectively evaluate. Of course you weren't aware of anyone having anything less than "complete trust" in your boss because of it; why would someone who felt that your boss was favoring those who drank with him tell you? People who perceive they're not part of a special social clique aren't going to go complain to the exact people who are then ones making them feel that way. It's not impossible that you're correct, but it's hard for me to imagine that someone would feel comfortable sharing negative feelings about the boss with you when you seem so confident that no one could possibly have any issues with the arrangement; if I were in that situation, I can't see myself even considering trusting how I really felt about it about it with a coworker who couldn't seem to even conceive of those feelings on their own.
I can see how I could have been in the dark about someone's true feelings if they weren't in a "clique" that I was in. In my exact case though, I felt like the 'clique' wasn't just the people who liked drinking at the bar, because we also had lunches including those non-drinkers, zoom happy hours (2020!), etc. I mean, drinking alcohol itself is optional even at a bar.
You're right that decision-making shouldn't be done by a committee delineated as "just the people who drink together" -- and that could be a real risk if this type of thing isn't being carefully considered.
However, the only guarantee against it than I can think of would be the far worse policy to simply ban all socializing after work, because the same thing could happen if 4 people took up after-work hacky sack and the others weren't interested.
I also totally understand that there's no requirement to drink at a bar; I just don't typically find them to be very fun environments. At least in the ones I've been to post-work hours, they tend to be loud and crowded, and I've always assumed that the only reason people tolerate them as a place to socialize is that drinking makes those aspects less annoying. Imagine if all your coworkers really loved comic books, and after work they always went to hang out in a comic book store, and you had no interest in comic books; you certainly could still go, but without actually being interested in what the venue provides, it's just not going to be particularly fun to hang out there.
This can be difficult to wrap your head around in the US because we don't have social rules anywhere near as strict as Japan and so empathizing with the experience is difficult.