I think it's kind of a German thing.
Even as a German, I sometimes struggle to connect with others for many of the reasons FirmwareBurner mentioned.
I made some good friends through a sports group I attend, which also organizes regular social events outside of training.
But aside from that, many so-called 'low-key social spaces' tend to feel like they're 'only for groups' Not because of any official rules, of course, although some places do advertise things like 'bring your friends', but in practice, you usually see people there in pre-formed groups. These aren’t necessarily close friends either, sometimes it's coworkers or people who already know each other through other contexts.
Those groups tend to stick together, and it's rare for outsiders to be included unless you already know someone who can introduce you.
>I think it's kind of a German thing.
Nordic, Scandinavian, German, Austrian, Swiss, etc any culture where personal space is valued and talking to random strangers/outsiders in public not the norm.
>but in practice, you usually see people there in pre-formed groups.
Because most people by the age of 25+, or whenever they graduate college/university already have their core social circle solidified and don't have the time, space and energy to seek or even let newcomers in. If you move into town after that age, you're gonna have un uphill battle to squeeze yourself into groups no matter how sociable you are or how good your bouldering skills are unless you run into other similar loners looking for friends, then you're in luck.
It's just the way it is, it's not your fault and not other peoples' fault , hence why I dislike such broad stroke one size fits all advices like "just do bouldering bro" as that's missing a lot of the context and variables that relationships are built on. You should choose activities based on what you most enjoy to do, not based on which others tell you leads to make relationships.