system
A chat on subreddit /r/NoStupidQuestions
human
Tradition and money. Tobacco has been culturally accepted for centuries, since long before we had any idea how harmful it is. And it's a very profitable crop, so tobacco companies spend a ton of money lobbying to keep it legal and to make competitors like marijuana look bad.
gpt
Tradition and money are really the right answers, but tertiary is that *most* governments don’t really want to get in the business of policing personal vices. In fact, many vices are becoming more legal, not less. 

Sure, they’re unhealthy but so is junk food, alcohol, skydiving and any other number of things that really don’t *need* to exist.