I think it's because we don't smoke as much as tobacco smokers. A smoker smokes like, what, 10-15 smokes a day, sometimes a full pack of 24 smokes. If you take a cannabis smoker who smokes 24, cigg. sized joints a day, I think they would get used to it.

I'm guessing there's about 1.5-2g's of compacted tobacco in each smoke, so do the same with weed everyday, I think you would become very used to it and not cough anymore.

I couldn't imagine smoke 24, 2gram sized joints everyday.