It hasn't been tried. And, often times, the objection come from those who want to demonize for what should be, and often is, considered a health issue. Now, consider men are much more stubborn when it comes to going to the doctor, and men are more likely than women to die from a terminal illness, often because when they finally did go to the doctor it was just too late. Consider the stigma against seeking help for psychological issues that keeps people away from therapists and psychiatrists?
Now, if someone is obese, and they are buying into this "they should feel ashamed of themselves" nonsense, do you think they are likely or less likely to get help with? And, yes, people do often need help. Help with learning about diet and nutrition, exercises that help them meet their goals, and any unique challenges they may be facing.