Why can’t people quit smoking/other addictions?

OK I am not perfect in the least, far from it, But i personally beleive quitting something is easiest things in the entire world.
I firmly beleive that if you cannot overcome and addiction from sheer brainpower then you are a bad human being.
My gf smokes and shes trying to quit for me but i tell her she cant, it wont ever work, she says she really wants to quit but i cant beleive her because i think if you really wanted something bad enough it wouldnt matter that your addicted to something.

idk if its pessimistic or what but in my brain, if i needed to stop smoking it would be that simple

how do you quit smoking? simple DONT SMOKE ANYMORE CIGARETTES

I dont understand it, perhaps its just my own philosphy but i would bet my life that i could smoke 5 packs of cigs, shoot up heroin crack and crystal meth and the next day when offered any of it I would just say no thanks. I beleive it is impossible for me to become addicted to something.