In Japan it wasn’t the case, it was straight up just people bicycling on the sidewalk in-between people walking. They just somehow figured out how to do it in a way that doesn’t disrupt the pedestrians, and I am still not sure how.

It does, people just don’t openly complain, much like any other problem - they either put up with it and say しょうがない or they complain in private or (often anonymously) online.

I am one of those annoyed people, I want bike lanes and I don’t understand why a place with so many riders is so anti-bike (aside from understanding the natural societal inclination to think that anything worth doing should be difficult, and made difficult if not).

I'm a biker and I want bike lanes too, so I don't need to dodge pedestrians. There is some bike infra here, but it's not nearly enough. And there's too many of those stupid painted "sharrows" that no biker with any sense uses because it's either next to too-fast traffic or blocked by parked vehicles.

I think the problem is just the politicians: they probably use cars and never bike or walk in these places.

I think the thing that makes it work the way it does is, as you say, people don't openly complain. They just get out of the way, or ignore the bikers (I prefer the latter as long as a bunch of them aren't blocking the whole sidewalk). In America, they'd be going full Karen and intentionally antagonizing the bikers, or trying to start fights with them, etc. Here, people seem to just recognize it's an imperfect situation, and put up with it instead of making a big deal out of it.