I’ve noticed this a lot. One problem is that there is no standard of citing primary sources. What often happens in medical literature is akin to a game of telephone. It might start out with a mere hypothesis or a careless mistake in reporting results, but what happens is a citation cascade where false or unsupported information gets repeated so many times it establishes itself as “fact” in the medical community.

“Citation is both an impartial scholarly method and a powerful form of social communication. Through distortions in its social use that include bias, amplification, and invention, citation can be used to generate information cascades resulting in unfounded authority of claims.”

Some of this could be fixed by just requiring everyone to cite a primary source. Peer review also is presently inadequate for whatever reason. There are tons of ridiculous errors even in high impact medical journals. You can find discussion sections that conflict with results, mislabeled anatomy (especially if it’s a clitoris), quantitative results that are obviously impossible based on methodology, and even just made up BS. Every now and then, someone will publish a completely made up article as a joke, just to show there’s a clear problem here.

There are also problems with misaligned financial incentives. Researchers may doctor results just so they get paid.

There are also issues in the surgery literature where the entire evidence basis will be experts evaluating their own results. Why do these experts publish outcome studies in the first place? To establish authority/notoriety. Obviously no one is going to publish bad results. There is no incentive to publish outcome studies that are actually representative of the average standard of care patients receive. The evidence basis in surgery gets a lot better when devices are involved, but there are still issues.