Wait: if tobacco use were actually the cause of psychosis, wouldn't we expect to see a dramatic reduction in the prevalence of psychosis in the United States, given that the smoking rate in the US has declined by over fifty percent in recent decades?

As far as I know, there has been no such decline.

What we have seen is a large increase in the incidence of depression, and tobacco has been persuasively shown to have antidepressant effects. Nicotine is, of course, a powerful stimulant, and tobacco also contains monoamine oxidase inhibitors (which also have antidepressant effects).