Why do some parents who smoke, not stop when it starts affecting their kids? ?

[Summer{J'sgirl<3}ILYt]

Like if they've been smoking too much in the car or house and the kids are starting to cough and get sick. There's some parents who choose not to quit even when they know its starting to affect the kids. I know its addicting but if their kids are more important than those cigarettes, it shouldn't be that hard. It's almost like a lady who's addicted to alcohol and still drinks when she gets pregnant. Dumb...