why do people feel the need to buy expensive cable

There are certainly some people who buy expensive cable for the reasons you state, but others who can actually hear what they're buying and are confident in their ability to assess the benefits the cable brings to the system and that the cost is appropriate for that level of upgrade. When I had my big Naim system, the benefit of the $1000 HiLine IC over the one supplied with my CD player was a very significant upgrade and much less expensive than upgrading my amplifier or CD player at the time. It's all about what you can actually hear yourself. Don't assume others can't because you can't. Some say they hear the difference between the two sides of the Linn felt mat. I don't, but I don't presume they're deluding themselves just because I don't hear it.

Why not i buy what sounds best.

Do you really believe that the cables make little or no difference? Would you consider a Radio Shacke receiver and bookshelf speakers with their factory supplied cables able to match an all MBL system with Cardas Clears? The system is only as good as its weakest link. I do studio mastering. The difference in detail and accuracy between cheap cables and reasonably priced high end cables is not subtle. It's true that the improvements shrink as the prices rise but isn't that true in every field. Is a Porsche 911 really five times better than its cousin the VW Beatle? Is the Porsche 918 really 10 times better than the 911? Is the the Bugatti Veyron three times better than the 918? The answer is an emphatic "no" if all you could afford was the Beatle in the first place.

One should to match the qualities if ones system with the quality of the cables to be owed. If you don't you will only be strangling the reasons you bought the equipment in the first place.

Don't buy Audioquest Wild Blue XLR interconnects to connect a $500 preamp to a $600 power amp. Match it to something like the Red Rivers. That way your system will be balanced in terms or performance and sonic quality and you won't have to worry about the rest.

I hear notable differences between stock cables and the brands I have used and/or tested in my system from Stealth, Sainline, Audioquest, Cardas, Kimber, Nordost and Shunyata. Every one of the above cables has made a difference, almost without exception for the easily discernible betterment of the overall sound---some raise the bar as much as an upgraded component, others provide a certain house sound to the proceedings.
That being said, I will probably never be convinced that a single cable should cost as much as I am willing to pay for a component.

In order to get the benefit of good expensive cabels, the rest of the system must be equal good.I think you can only get the full potential of your equipment when you use eg. equal good cabels. The same as with decoupling. The weakest link in the chain sets the level and cabels are also a component.
At a certain level of refinement, it becomes much more obvious what changes in the system, does to the sound. It is possibel to get a concert into your lisiningroom but everything in your system must be in ballance and at a very high level.
Better cabels helps the system haveing a open, airy, natural clear 3D view without artifacts at all.
Its like looking at a 300" widescreen, its fantastic but the flaws are easily seen, therefore everything must be perfect or you better stay with the 50" screen and save the expensive cabels.
Getting an ultra highend system to play correct and find the full potential takes years and thats the art about it. The closer you get, the more you fell the transformation of feelings from the musicians.