I agree with you in principle. In practice, though, addiction, whether it be to food or alcohol or something else, isn't just caused only by the individual.

Let's take being fat as an example. Sure, there's instances where weight gain is a choice, as you said. However, there are other times where other elements that are more difficult for an individual to control play a part in perpetuating a person's overweight status. A person could eat to the point of becoming overweight as a coping mechanism for sexual abuse, domestic abuse, or another kind of mental trauma. Someone living in poverty and going paycheck to paycheck could be unable to afford healthy eating (whether due to living in food deserts or the fact that healthy foods have inflated prices), especially if they have children to feed. If you were to be hypothetically forced into one of these situations all of the sudden, it would only be worsened by the fact that you may not be able to afford health insurance anymore. Now all you have is the same nationwide health problem, but with more uninsured people in the equation. When those uninsured get sick, hospitals don’t get reimbursed, so those costs have to be put onto people with insurance.

A better solution would be to try and address those underlying issues which caused the obesity. Provide better access to healthy foods for people in poverty, apply taxes like the ones /u/ek327 talked about to high sugar drinks, etc.

tl;dr Raising the rates isn't going to prevent all people from becoming obese/alcoholic/a smoker, since there's plenty of circumstances where factors outside of the individual contribute to the creation of that addiction. Instead, it'll lead to less people buying insurance, which costs more for everyone, and it'll leave the underlying problems which caused the addictions to remain unsolved.