I thought the same; at least in the kidney stone story, the data wasn't balanced: treatment A was assigned a lot more "harder cases". Either the trial wasn't randomized or the data set size wasn't big enough.
Unless you are God. You will never be able to even properly know what to factor in. Actually doing the experiment and analysis is exponentially harder. It's like saying well who cares about p=np, if you want to decrypt Aes without the key just make a super fast computer.