It goes beyond that. Just because we know a substance is essential does not mean we know an effective method of replacement, or whether a proposed method is both safe and effective. The biochemistry may tell us one thing but theraputics requires many other separate functions.
If it has not undergone appropriate scientific study, it is never any more than a belief.
Vitamin therapies often require megadoses of materials known to be required at very low levels. The rational for megadoses is often the belief that they will travel to the sites of activity in adequate levels to be effective. Sometimes the site of activity is known, sometimes it is not.
The way of proving safety and effectiveness is well known, a well designed, controlled double-blind study, like drugs have to go through. A process which is cost prohibitive for many substances and is not attempted for most vitamins, minerals and supplements.
Human beings get their beliefs from a variety of sources, some more reliable than others. They can be right or wrong. They can be rational or irrational. But they are unproven.
Great medical breakthroughs have come from medicine men and shamans around the world, and they still have required scientific research to become accepted as other than belief.
There are many cures waiting to solve medical problems which humans face, if they could just solve how to make them pass the blood brain barrier or other obstacles which our bodies provide. What works in a test tube or a petri dish does not always work in our bodies.