Smoking habits are vastly different between the two. THC has strong inhibiting effects so must people will usually smoke smaller quantities (often just a quarter of a gram) and will probably do it as a way of unwinding after work or share a joint with friends when they're hanging out. Nicotine on the other hand isn't inhibiting and if anything can give a little boost. Because of this someone can smoke a whole cigarette and go about their day at work. Often times taking multiple breaks during the work day. Tolerance also builds up very quickly and there are significantly stronger addictive effects which often leads to people smoking several cigarettes a day even when they aren't getting the effects while the practice of taking tolerance breaks is pretty common even with heavy weed smokers since there isn't any real withdrawal. Sure there are people that smoke multiple joints a day, but there are also people that smoke a pack or more of cigarettes a day and they're able to stay much more functional while doing so which means that there isn't the negative feedback you'd get from being stoned all the time.

All of that would hold true if both substances were equally toxic. However studies show that weed is far far less carcinogenic than tobacco. It's not healthy to smoke anything, but what you smoke and how much does matter.