The reason is because their skill set reaches hundreds of thousands of people while a doctor/engineer can only reach maybe a thousand. The rationalization is they can reach a massive amount of people and their effect on the world is much broader (regardless of having a higher impact).
I honest have never seen a Dr Oz show but I know he has a broad audience therefore he should have a much much higher level of scrutiny than any other doctor.
Regardless the OP's question is still valid, if only the goal is to improve upon the output of the science.
While I certainly don't subscribe to anything so foolish as wholesale dismissal of the entire establishment, it's not as if every medical professional came out of a factory with no bias, flaws, or even fully developed critical thinking skills.