Your analogy is false.
If you opt for it, then yes of course, abstinence-only sex education is a sure way to stop ten pregnancy.
Similarly, if you opt to change your lifestyle and start living healthier, and follow through, it will work.
If someone tries to force you, of course it will fail (in both of these cases). There is no way to stop someone else from being fat. We can only change ourselves.
> We can only change ourselves.
Being fit and maintaining the normal weight is not the same thing as reaching a BMI of 50 kg/m² then losing it. I've seen fitness influencers take up that challenge only to find that "changing oneself" is significantly harder than it used to be when they were fit.
It's not even a case of internal/external motivation. Even internally motivated attempts often fail. You can always claim that they just didn't want it enough or lacked willpower, but it still reduces the effectiveness of the approach. You can claim any approach for solving a problem works if you reject all the cases where it doesn't.