So what? If I could take a sugar pill that guaranteed I feel comfier looking at my screen, nobody can tell me it "doesn't work". I'm not trying to optimise my life, I'm trying to have my eyes feel better.
If somebody is "manifesting" themselves a sleep aid, I think they'd just call it meditation and everybody would more or less accept that it probably works for that individual. Maybe you'd have a few people with severe autism who start arguing on online forums about the scientific evidence behind meditation, but that's just them being them.
The placebo effect is a real, measurable mind-body response where belief & expectation can change your symptoms or how you feel. However, it does not directly alter external reality. Manifesting claims your thoughts or intentions can cause _outside events_ to happen, which has zero evidence to support it.