I doubt that James Randi has been able to demonstrate
I don't think he tried.
The point is that while some psychics, mediums etc really believe the stuff, the majority are deliberate frauds. Houdini as well exposed a lot. They are deliberately using Stage Magician tricks. Hence people like Houdini and Randi have known how to devise experiments to catch them out even when scientists have been fooled by poorly designed experiments.
Randi's foundation wants people to use scientific method and scepticism, not accept claims at face value.
A negative result doesn't infer the non-existence of something
True, but in the case I'm talking about people are making claims something exists (homoeopathy, telepathy, talking to dead, remote viewing, dowsing etc) and results of properly done experiments match placebo or chance.
They aren't talking about scientific proof of love, justice, mercy, hate, creativity, inspiration etc. The fact is that if the phenomena above did exist vs number of supposed practitioners, there WOULD be evidence. The results are often "positive" not negative. Discovering how the fraud or deception is done is a positive result.
Placebo and Nocebo are positive results too. That's outcomes different from doing nothing or chance. We don't know what causes Placebo and Nocebo effects, but if you believe something will harm you, in significant number of cases it does, and if you think it will help, it may do, though there is no good reason. Hence doctors are being warned to be careful with describing prognosis and describing efficacy of treatments. Certainly there is a mystery there.