Are most advice in self-help books not good productivity/happiness advice?
The problem with both is that people who can follow them for effect already are following them. So in practice, diet and exercise is what I call "fuck off advice": the person you give it to, should they believe it, will try and try and not see much effect, and eventually fail - but the process will take 3-6 months or more, during which they won't bother you with their problems, and at the end of it, they'll blame themselves for not being good enough, and perhaps even see you as a good person who tried to help.
The problem with both is that people who can follow them for effect already are following them. So in practice, diet and exercise is what I call "fuck off advice": the person you give it to, should they believe it, will try and try and not see much effect, and eventually fail - but the process will take 3-6 months or more, during which they won't bother you with their problems, and at the end of it, they'll blame themselves for not being good enough, and perhaps even see you as a good person who tried to help.