Lots of things are effective that are also bad solutions, especially in a different context.
So you’re right, shame works. But, for the sake of argument, if it leads people to suicide, eating disorders, or depression, while obesity is no longer technically the problem, you’ve just created another problem.
Shame works for Japan (for now, at least) because they are a wholly different society than ours, based on a wholly different set of morals and upbringing.
But is a Japanese salaryman happier than an obese American? Most are miserable, for most of their lives, and simply don’t talk about it to anyone outside their close circle, if at all. Remember that mental health in Japan isn’t really a thing that gets discussed very much, by doctors or anyone else.
It’s just trading one thing for another.
We’ve learned a lot about reinforcement learning from training animals, and specifically that positive feedback tends to lead to better results long-term, even if negative reinforcement may lead to quicker changes in the short-term.
I’m not convinced the same ideas don’t apply to mammals like us.
Moreover, what gets lost in the shame / blame game is the concept that people can work really hard at changing themselves and still have a long way to go. That is, if someone is 200 lbs overweight and has spent the last two years losing 100 of those lbs… someone who sees them in a “shame” society would still shame them, with zero context given to the fact that they’ve had a massive success over the last twelve months that should be celebrated even if they’re not where they want to be yet.
Without positive feedback, it’s exceptionally difficult to “stay the course,” and that is the point. Shame only gets you so far.
Supporting someone on their journey to a healthy weight gets them much further, and if you genuinely care about someone, that should generally extend to their mental health as much as their weight, imho.