Because other people vilify it. As long as you're in a crowd of people saying the same thing you must be right and good. It doesn't really matter if you are yelling at people for smoking or for supporting the wrong party or football team. As long as you join a crowd and shout when they do.
There are of course serious downsides to smoking, but the vehemence is out of proportion to the effect it can have on a casual passerby and no one wants to ban people from harming themselves or skiing would be illegal.
Some posters ask "but you don't get so hot about people drinking" and they get flamed because... well you can't shout about that. Everyone has friends or relatives who drink so it just can't be done.
My town center is off limits at night time because of the violent drunks. In the morning someone has to clear up the broken glass and vomit, but hey that's just being sociable and drink never hurt anyone right?
If your best friend regularly gets in the car after a drink you can't say anything can you - that would be rude. If he ploughs into a bus stop full of people that's just bad luck. But "oh god I saw a smoker in the park yesterday! he was 50 ft away, but I wanted to kill him". The hostility is just over the top.
The media don't help because when they print research results they don't explain them properly. When you see that "millions died of smoking" that won't be what the researchers said. "contributed to" would be more accurate. Those millions are the same people who died of obesity and the same people who died from heart disease, liver disease and so on. Because all of those things contributed to them dying earlier than they might have otherwise
After a bit people start making stuff up about it. Like the bogeyman gains more teeth and claws with the retelling.
There are people on MN who believe you can catch cancer because someone who visits you is married to someone who sometimes smokes and it gets passed to you on their clothes.
One poster had a sealed package delivered in a van in which someone had smoked when it was not being used for deliveries and she had to destroy the items in the package in case it infected her family.
It's not even helpful. Tell kids it makes them smell bad - which is true. Tell them it makes them out of breath walking up stairs - which is true. Tell them it greatly increases their chances of getting serious and terminal illnesses - which is true. Even tell them that they won't be very good at sex if they get out of breath all the time, but don't snatch them away from the road because a truck went by with a smoker in it. if they see people being irrational about it then they will dismiss all the warnings as lies and propaganda.
I wouldn't be surprised if most smokers now were the old people who started back when the same mob said that non-smokers were anti-social/miserable gits. It's not easy to stop once addicted.