I think we understand the causal mechanism pretty well. it is just laypeople struggle with binary thinking vs probability.

The reason why only 20% of smokers get cancer is similar to why a person doesn't get cancer after 1 cigarette.

This is only counterintuitive if your default thinking is that smoking=cancer. In reality, there are a lot of variable chemical and biological processes involved, but ultimately it ultimately boils down to a cumulative risk, not guarantee.