why do people feel the need to buy expensive cable

Two different ways to tweak in my opinion. Very much a matter of preference and goals. However with good quality modern digital processing technology its very quantified, measured and reliable to an extent not possible otherwise. Like most things in audio many ways to slay the beast effectively for someone who knows what they are doing.

"Using exotic cables to achieve a desired sound is not cost effective."

That statement is actually an excellent example of what is known as a Strawman argument. Misstating the opposing side’s position. It’s a common ploy of those who object, perhaps strongly, to expensive high end cables to portray cables as some sort of tone control device, as if the high end cable manufacturers are engaged in some sort of global conspiracy to manipulate the naive and gullible audiophile. And that audiophiles purchase high end cable to control some audio characteristic or another, you know, tame the highs, get more bass frequencies, etc. Furthermore the determination of the cost effectiveness of a given cable or a given anything actually would require a formal system engineering study, not an off-the-top-of-the-head opinion.

i'm cuckoo for cocoa puffs

Two words!

SNOB APPEAL!

Too many here are stating things definitively so as to preclude any debate without the first hand experience to back it up. 

I used to be in the camp that simpler was the better way to go and as long as one used a high enough purity with the conductor material, and use the least offensive dielectric, then that was all was needed. And, indeed, there are cables that fall into that category that fill the need without resorting to deep pockets.

It was only when I got some discounted, higher priced cables that I realized I was gravely mistaken in my belief. I had to hear it for myself. Not only was I wrong about the premise regarding construction, I was also wrong on the monetary amount needed to achieve my present state of audio satisfaction. 

Considering what I've spent, slowing bleeding my wallet with various cables when I could afford to try something else, I just should have gone for the best I could afford and save money in the long run.

Lesson learned.

All the best,
Nonoise