Why can't I smoke when I'm breast-feeding? I see many mothers do it and their babies turn out to be fine, will I hurt my baby

Yes. Start with an increased risk of sudden infant death syndrome, add 2-3x incerase in bronchitis& ear infections & other respiratory issues. Add decreased growth & exposure to nicotine in the milk, short of this you will teach this child to smoke as an adult. I am not aware of any health benefits of smoking to you or your child. At least you are breast feeding, thats good.
Not healthy for baby. Yes many babies turn out fine but that does not mean it is optimal for your baby. It exposes your baby to unnecessary toxins. Now would be a great time for you to quit, let the baby be your motivating factor! it isnt healthy for you either. Also, many illnesses and conditions can be seen at increased rates in infants who are living in a home with a smoker. So it will affect baby later too.