I think the difference is that smoking is also horrible for the smoker, and many want to quit, but can’t very easily. People don’t want to stop eating meat, driving their SUVs, buying cheap shit on Amazon, and flying to cool vacations. Stopping doing those things doesn’t benefit them unless everyone else stops too, and unlike smoking, we’ve made little progress in the last few decades (other than the progress unlocked by technology that means it’s immediately beneficial to the one making the change, like solar), so I can’t blame anyone for thinking that giving up meat to save the planet is stupid since their entire lifetime of lived experience tells them that the vast majority of their fellow humans are not going to do the same.