puritanism and moral authoritarianism. People love imposing their morals on others, and most are taught from a young age that drugs other than alcohol and caffeine are bad
But if there was one thing I would consider trying, it's psychedelics. It can help you think outside the box, apparently has some link to treating depression, and they're not particularly addictive (other than an adrenaline junkie type of addiction, which can develop with anything).
Point being, I don't do anything of the sort. But it baffles me that people are incapable of allowing others to do things that they personally don't want to partake in.