why do people feel the need to buy expensive cable

the reason...for the same reason most of us buy or want to buy expensive components. The desire is to obtain great sound. I spent on equipment at first and only thought about cables afterward, having been exposed to the idea that good cables made a significant difference, by reading about it here, for the most part.
I will add that it is easier to spend a couple of hundred on a cable than coming up with the money needed to buy better components, thus there is appeal in that sense. The OPs "expensive" is still undefined, if it is thousands then I have no idea why some people do it. Perhaps they have spent much, much more on their equipment and this the final tweak.
I did find that certain cables made a meaningful difference in sound quality and others did not. The price didn't seem to be the defining factor.
But I have not spent thousands on individual cables, so my experience is limited by my cheapness, about $500 and mostly less. Most normal, non audiophiles would consider that much spent on wires to be truly insane.

The usual reason audiophiles dismiss more expensive cables in a-b comparisons is because to be blunt, their systems aren't capable of clearly revealing the differences.

To summarize, if you can't clearly discern the differences between cables it is probably that your system isn't at the level of the cables, or your maybe ears aren't sophisticated enough to hear the differences. Really...seriously?

10-27-15: Tonykay
To summarize, if you can't clearly discern the differences between cables it is probably that your system isn't at the level of the cables, or your maybe ears aren't sophisticated enough to hear the differences. Really...seriously?

I hope you get the answer you want so you can live on comfortably in delusion. Something about bliss?

I like yuppies. They go buy very expensive stuff, soon after that they do it again, and we have a great choice of cables for 25% the initial cost.
But not all people who buy expensive cables and components are yuppi audiophiles. Some audiophiles on moderate income actually save up big time to get what they want, including new. I don't mean $10k interconnects, $100k amps and $200k speakers. That would be hard to save up for on moderate income. I mean $2k interconnects, $20k amps and $40k speakers. Yeah, most of them buy used.
Besides, there are rich people who are true audiophiles. They just buy what appears to be the best to their ears.
I don't envy them much - they have nothing to upgrade to. Or almost nothing.