I get the impression the variable is not social drinking but social adaptitude. I do want your opinion on the matter, however. Every native Japanese person I have known is incredibly reserved. As an example, I had a Japanese coworker, and I did not even know they had a wife until they corrected me in conversation. Maybe it is the subtle aspects of the language (my Japanese is still not great)? I am looking to establish an EOR in Japan and eventually move there with the goal of citizenship, but I've been holding off until I understand the language well enough to get by without outside help.
People in Japan tend to avoid talking about personal matters in the workplace. In my twenty years at a university here, I have met almost none of the spouses or partners of my colleagues, and there have been quite a few colleagues—including ones I worked closely with—whose marital or relationship status I never knew. Faculty social gatherings never included spouses or partners, except informal events organized by the foreign faculty. My impression is that the situation is quite different at universities in the U.S.
The exception was children: My Japanese colleagues—both male and female—often mentioned their kids, and at the campus where I worked until COVID it was not unusual for faculty to bring their children to the office during school vacations. The faculty all had private offices, and there was no problem with children hanging out there during the day. That would be much more difficult for office workers, though.
Another reason why Japanese people you meet seem reserved could be the language barrier. If they aren’t fluent in English, they may seem less outgoing than they would in their native language.
Good luck with your Japanese studies. Knowing the language makes things much easier here.
