That's actually a big reason why cigarettes are so notoriously difficult to quit. Yes, hugely addictive chemicals, but its the psychological oral fixation that is hardest to break. You can wean down off nicotine (or THC) but your body and brain have been trained to do the thing with your hands and mouth, repetitively, all day every day and unless you can replace that with something else or resist the habit, you'll end up going back to it because you don't know what to do with your hands (and mouth).