Smoking to lung cancer has a very direct delivery mechanism, inhaling tar into the lungs. The effect size and sample sizes are big. The hypothesized mechanisms here - unfavorable social comparison is far more tenuous, and the sample size here is number of universities- not number of students.
These 2 are vastly different situations.
To give an example. Establishing causal effect between nicotine and lung cancer is an open question, even as the causal effect of smoking on cancer is very clear.