I may never have smoked but have got a history of addiction to hard drugs & alcohol. Yes I managed to quit the drugs & yes making a simple choice was a part of that. 15 years on though it is sometimes still the most difficult thing to do, I would love nothing more than to get high again. I am lucky though in that I have managed not to, or to misuse prescription medication to a lesser degree instead. But there are so many complicating factors & involvement of various parts of the brain and body that scientifically, no it not as simple as making that choice. Yes, for some people that is enough. If you are in that group consider yourself lucky. For others it simply isn't.
Add in to this, as @twinklify noted, dual diagnosis MI & addiction is common, extremely common. Being in a psych ward atm, I am the only non-smoking patient here. Has been that way since I was here. Could count on 1 hand the number of non-smoking patients that I was here with in over 3 months earlier in the year. Most people who have tried to quit, some desperately but haven't been able to. It hasn't been through lack of making the choice to. Same principle for drugs, alcohol & smoking. For many, addiction to substances is as much as illness as my MI is (though many think that is a figment of my imagination too so can see why people can't get that).