Quitting smoking is difficult. Cigarettes contain nicotine, nicotine is highly addictive, and that is what makes it, at a biological or physiological level, very difficult to quit.
The other reason that quitting smoking is so hard is that smoking is habitual. You, smokers, develop patterns in their daily life of where cigarettes become very important. I’ve just had something stressful happen, I’m gonna have a cigarette. I’m having a cup of tea or coffee, my routine is to have a cigarette. When I go out for a walk, I’ll have a cigarette. So cigarettes become part of one’s everyday life, and nicotine, and the nicotine addiction, makes it much harder to quit cigarettes.