There is evidence that some additives enhance addiction and that others suppress the side effects from smoking (coughing, burnt throat, feeling ill, etc.) so that when someone quits smoking they have essentially stopped treating themselves for all these side effects, which then appear and make the person feel miserable. Smoking instantly relieves symptoms again (for a while):

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2040350/

"Our findings indicated that more than 100 of 599 documented cigarette additives have pharmacological actions that camouflage the odor of environmental tobacco smoke emitted from cigarettes, enhance or maintain nicotine delivery, could increase the addictiveness of cigarettes, and mask symptoms and illnesses associated with smoking behaviors."

So it appears that although nicotine is mildly addictive, commercial tobacco was engineered to suppress the side effects of smoking which essentially made it much more (technically) addictive. Hence quitting tobacco is much more difficult than quitting nicotine because you are also quitting the additives which are suppressing cough and hiding the feeling of illness smoking causes.