It's just simple math.
Look, I was a Tobacco smoker, and the level of addiction you have with Tobacco is on another level.
I was smoking easily 20 cigs a day. I don't know anyone, even the most stoner of stoners, who smokes 20 joints a day. That doesn't even sound, like, logistically possible.
Obviously smoking anything is bad, but getting an order of magnitude or more less smoke HAS to be a big win.
Also nicotine. Nicotine even by itself is a problem. Don't listen to Zyn bros, nicotine is in fact evil and it does destroy your cardiovascular system over time.