They don't though.
If they did, they'd have the data to support it and they don't.
There are plenty of people who think vaginal douches, onions taped on to a body part, prayer, drinking breast milk or drinking their own piss cures cancer.
None of it backed up by any actual studies.
One of the latest trends is deep cold plunge baths. Loads of people saying it's cured their anxiety or depression. Zero evidence in studies to say that's true.
Cognitive bias and placebo effects are very real. People find the evidence they're looking for and discount the evidence that refutes it. Not even consciously, they just do.
That's why we have stringent studies, often double blind which continually find it's not true.
And before any woo manifesting people say the placebo effect means your mind is so powerful you can cure yourself if you believe you can.
No, that's not what the placebo effect is. .
The placebo effect is that someone can believe they feel better because they had a pill, or an IV or any other intervention. Particularly in conditions that can be influenced by psychological states like anxiety, blood pressure, low mood, and so on.
So in studies for asthma for example. The people receiving a placebo often report feeling they could breathe more easily but their peak flow would say no they weren't, it was just their perception.
Give a type 1 diabetic a placebo instead of insulin and they will be very unwell quickly even if they believed it was insulin.