Not all smokers get lung cancer. But with large enough samples, smokers turn out to be much more likely to get lung cancer than non-smokers. That's why we say that smoking causes lung cancer. It's not as if someone has actually watched a particle of tobacco interact with someone's lung cells to turn them cancerous in real time.
For one I'd assume its because contagions and physical damage are quite different. That's not to say that correlation between viral presence and disease can't indicate causation, but it is more clear when you are looking at the direct damage to lung tissue from inhaling hot cigarette smoke for years or decades.
Not all smokers get lung cancer. But with large enough samples, smokers turn out to be much more likely to get lung cancer than non-smokers. That's why we say that smoking causes lung cancer. It's not as if someone has actually watched a particle of tobacco interact with someone's lung cells to turn them cancerous in real time.