I don't know any of your friends, so it's possible that you really do know people like this.
My own experience (accumulated over a long period than you?) is that loads of people who smoke weed and cigarettes will try to quit cigarettes but end up continuing to smoke joints, for that tobacco/nicotine hit (right,
@moses ?) which makes it much harder to quit tobacco for good than it otherwise would be. I have never once met a person who took up weed smoking, late in life, for the first time after quitting cigarettes. That seems like an extremely fringe case.
Thankfully, the push to legalise weed means vapes and edibles are more and more widely accessible, so we are finally moving towards breaking the link between weed and tobacco.