why do people feel the need to buy expensive cable

It's indeed just a fraud, that is all there is to it. And a lucrative one.

It has already been done in a UK court of law: the cable seller lost and had to withdraw the product from his chain of stores.

Keep dreaming Geoff

I think I let them continue to rip off people like you. I have better things to do, and unlike in the US, there are no damages to be had.

See here for a more recent case: https://www.asa.org.uk/rulings/the-chord-company-ltd-a14-274211.html
The snake oil sellers keep trying.
See here for a US example, though not about cables: https://www.ftc.gov/news-events/press-releases/2014/11/sony-computer-entertainment-america-provide-c...

Not court cases, but effectively with the power of a court. Typically, in Europe the state tries to leave such things to the self regulation of an industry (in this case the advertising industry). If the industry fails to clean up a sector from misleading and deceptive practices, the state will step in. Here in the Netherlands, that has led to very expensive court cases for banks, for example.
So, aside from the technicalities that I had not remembered quite correctly, society has acted against these crooks.

Sure, plus the buyers usually do not belong to the destitute classes, so there is little sympathy for the victims. There are indeed more important things for society to worry about.

For those who want more information, from well known electronics engineer Douglas Self: http://www.douglas-self.com/ampins/pseudo/subjectv.htm
With quite a bit of bibliography for those keen to enlarge their knowledge.

Mr Self does not need ESP. He has a first class honours degree in engineering from Cambridge University (i.e the top 10% of his year) and then psychoacoustics at Sussex University. For years he worked with audio legend Peter Baxandall and a number of well known electronics manufacturers. He holds a number of patents on electronics design and is the author of a classic book on amplifier design. Geoff I cannot help it if you have never heard of him.
This is what he wrote on the directionality of cables: "Cables are directional, and pass audio better in one direction than the other."
Audio signals are AC. Cables cannot be directional any more than 2 + 2 can equal 5. Anyone prepared to believe this nonsense won't be capable of designing amplifiers, so there seems no point in further comment.

The quotation marks are his, to define the position he wants to argue against. His own view comes with the words Audio signals are AC.....
I am afraid I created some confusion. What I should have writen is:
against the view that cables are directional, Douglas Self answers ' Anyone prepared to believe this nonsense won't be capable of designing amplifiers, so there seems no point in further comment.'

Sure, and very good authority it is.

Civilization has advanced over the last few centuries by applying rigorous standards to distinguish between nonsense and truth. Some audiophiles and their obscurantist merchants like yourself (I originally meant Geoff, but it seems Audiotroy wants to join that crowd)  have worked hard to stop that progress in the world of audio.

However, five times 0% is still 0%.

Well, in advanced economies we have had a roughly 2% per capita growth of income over the last century or more, even though world population has increased enormously. We have more than doubled life expectancy, we have eradicated many chronic diseases, and we have increased levels of education enormously, just to mention a few metrics. And whereas the benefits of all this were originally limited to Western Europe and the US, this pattern is now extending more and more all over the globe.

The argument is that the onus of proof is on you. I agree.

I would guess closer to 90%