Because we've decided it's nonethical for healthcare professionals. Any doctor knowingly prescribing placebo and lying about it could get in some major trouble. Like a couple decades ago I heard a psychiatrist mourn the loss of disulfiram (antabuse) implants from their treatment arsenal; it worked very well as a placebo but research didn't show a clear improvement over placebo, so they could no longer use it.

I am kind of glad that non-license-needing wellness consultants can still use the placebo effect for good, even if it is sometimes predatory and sometimes outright dangerous.