This is the crucial limitation of RCTs for this category of problems - they are big, they are expensive, they answer only a small number of questions (very often only one) and if you don't see an effect, often can’t tell you why - a significant drawback vs mechanistic approaches.

A drug trial that shows clear evidence of efficacy is a positive demonstration that the drug can work. A trial that does not, *cannot* show you the drug can't work - the results only hold for the specific protocol tested.