I don't really understand it either. Added stress? Maybe they drink too much too? The real heavy smokers I know who are out of their 20s are all heavy drinkers.
Personally, I was fit until I started taking psych meds that can cause weight gain. My good habits fell apart, I stopped working out too, and my sleep became awful.
Not too surprising what the results were

So there are factors that affect hormones and such that regulate hunger, blood sugar, etc., but ultimately there's still some basic fundamentals everyone has to adhere to in order to stay in shape (eating less calories than you burn, ideally some level of exercise).
Going to do keto for a few months and lose this dad bod. My dad also caught the diabeetus so I need to be careful.