If people only used the medicine that was actually fully and completely scientifically understood and proven, they’d exclude most of modern Western medicine.
Only confused people reject known beneficial practices on the basis that “science” (let alone the patient themselves) completely understands and/or proves them.
Not only do we not understand our own conventional medicine, but few scientists ever seriously make any attempt to engage spiritual healing scientifically. And proving things are false is generally even harder than proving something is true. Skeptics that just hear of anything spiritual and immediately yell “hoax” are being extremely unscientific.
And a healer who has and shares ideas that don’t make sense (or are disprovable) in literal scientific terms, isn’t necessarily doing things that aren’t helpful. The explanation doesn’t have any effect – it’s just comforting to minds that want to feel like they have a logical explanation. And that comfort (or upset if it’s missing) surely also do have effects all by themselves.
And, of course, conventional Western medicine is FULL of comments about how stress effects health, and its charting of the nervous system clearly shows that all of the body’s systems are controlled by the nervous system, which ties straight into the brain – the communication network and the brain are made of a connected network of neurons. The placebo effect and several other documented mind/body causations demonstrate that even ideas can have direct effects on disease and many other aspects of ourselves we traditionally think of as fixed or physical.
Spiritual healing then clearly can and does have physical effects. But what thoughts/emotions/etc have what effects is not a mechanical “do this, get that” clear thing.
Basically, “skeptics” just really don’t like spiritual thinking. Perhaps because their over-analytical habits of being have suppressed that aspect of themselves, and they have conditioned responses that tend to keep them from confronting that.