I've noticed this! What's up with that?
Trying to manage moods all day, only it's on the front burner with schizophrenics and failure means hospitalization. So it's coffee to feel alive again after the deadening effects of the antipsychotic medication. And it's cigarettes to "even things out" as my son says.
I also suspect part of it for some is a structure and ritual easing socialization. Mental illness leaves a person feeling very separate and dealing with other people plus internal stimuli can be additional stress. Having the simple structure of sharing/borrowing cigarettes and the tiny underground economy around the habit, the comraderie, is an artificial "tribe" to which they almost all belong. My son rolls his own and the purchasing, preparing, and smoking of the tobacco is a focal point of his day.
Anything that helps him get through the day, even if it shortens his life, fine by me.