Because public health has worked so very hard over the years to associate smoking with disgust in the minds of the public and to use shame as a tool to encourage people to quit.
It's been fairly successful - it has stopped a lot of young people from taking up smoking and it has encouraged those smokers who can be motivated by shame to at least attempt to quit.
The downside is that most of the remaining smokers, who usually have a long history of unsuccessful quit attempts behind them, come to view themselves as disgusting and shameful. This level of low self esteem really is not conducive to successfully quitting such a tough addiction. Or they develop a 'don't give a shit, fuck you' response, which is probably at least better for their mental health, if not their heart and lungs.
The other downside, which I think is what this thread is about, is that other people end up not only thinking smoking is disgusting (fair enough) but that smokers are too, and you can see in a lot of the comments here how that spins off into ideas of smokers being stupid, selfish, wasteful, filthy, of weak character, vile, 'infected' (even after quitting!), a waste of organs ... just a bit sub-human really.
Most long term smokers started as children and it doesn't take very long at all to get very heavily addicted. There is a HUGE correlation with both low income and with having a diagnosed mental health condition. It's the biggest cause of health inequalities in this country.
A lot of posters here don't care about any of that, or what would actually help smokers to quit. They're just stuck at 'disgust'. You can tell this when they start banging on about vaping as well. Never mind that it's made far bigger inroads into smoking prevalence than all the shame tactics in recent years, it looks a bit like smoking so it triggers the same disgust response.
To those posters: This is for you -
www.thedailymash.co.uk/news/health/put-cancer-in-e-cigarettes-say-non-smokers-2014021983765
Another very good example is the thread I started in Site Stuff asking if we could get someone on to a do a webchat about quitting smoking in pregnancy. I thought it was a reasonable request, given that at least 10% of pregnant women are still smokers at the time of delivery and that pregnant women are a huge core audience for this website.
www.mumsnet.com/Talk/site_stuff/2984138-Please-can-we-have-someone-really-sensible-on-to-do-a-webchat-about-smoking-in-pregnancy
Three YEARS since I started that thread, there still has not been a webchat. MNHQ made a few encouraging, non-committal noises every so often but it's clearly just a bit too icky and 'not nice' for them to want to engage with. I gave up asking in January of this year. Smoking prevalence in pregnant women has remained about the same over all that time. In fact I seem to remember it went up slightly.
And there were posters on that thread who were OUTRAGED at the very idea that women should be helped to quit smoking during pregnancy. They apparently preferred that pregnant women continue to harm their health and the health of their babies than be offered the help they need to quit. If they couldn't just quit 'for the sake of their babies' then there was no hope for them (or presumably for their babies) anyway.
The same type of arseholes turn up on every thread started by a pregnant woman who is looking for support to quit.
These attitudes are not unique to MN but it is particularly bad here and MNHQ share the blame for that.