Experiments aren’t cheap. Patients aren’t in a good position to evaluate if a product is working or not. This is especially true when they are emotionally or personally involved in the outcome - everyone believes they are an outlier.
Also, how does the market work if the customers die (because the treatment is a sham) before they can warn other potential customers? What stops me from peddling snake oil to every dying person, knowing that other dying people will never hear about how it doesn't work?