Earlier quoted context omitted.
You're putting the temporal cart before the causal horse. With or without drugs, mental health issues can be latent or unacknowledged for years. And without defending marijuana use, naïve -- even unconscious -- self-medication is a very simple explanation for what you're describing.
Yeah, I particularly like two counter examples. Example 1: We have prevalence of schizophrenia of 1-2% both in countries with extremely high use (Australia, 25-30% last year) and extremely low (Japan, Example 2: The countries with the absolute biggest rates of tobacco prevalence are hovering around 40-50%, the US is sitting at around 20%. Among schizophrenics, though, it goes around 75-90%. But funnily enough, you do…
The association is likely tied to self medication. Tobacco contains a low level monoamine inhibitor, these have some uses in psychiatry. (it is the main bit that is addictive - not really nicotine alone - harman and norharman, related to harmaline)