Because someone's opinion (and doctor is just someone unless he repeats something that he learned in school or from solid medical research) ... opinion is not knowledge and only knowledge can get us beyond pure luck. Knowledge is acquired through solid research. Which means large randomized studies with control group, blinded or double blinded.

And I'm not saying we shouldn't do more tests. We definitely should do more tests, and design them carefully, write results down diligently, and be ready to accept if they say that there's no effect because that's what most likely to happen with any substance that we test.