Giving someone Treatment X likely means they won't be pursuing Y, whether it be to avoid polypharmacy drug interactions, or simply because the patient prefers X's side effect profile to Y. Y may be the proven treatment, with worse side effects but better efficacy. Even if the doctors inform the patient "X may not work at all and may make it worse", the patient might still choose it.

"What's the harm?" you say. The problem is, without any threshold, there are countless Treatment X. Every single snake oil comes out of the woodwork.