IMO, the answer is pretty obvious. It's 99% about marketing. The latest cell phone, computer, TV, video game, you name it. It taps into what seems to be a natural human craving for novelty (gee, why do guys cheat on their wives), and marketing & advertising people know well how to exploit that. Then there's the desire to be the first on the block to have the latest/greatest gadget - witness the lines of idiots waiting to buy the first iPhone. And of course the ego aspect of owning some "holy grail" thing that's "the best" one out there.

The phonomenon is certainly not limited to guitarists, but they're a good case study.

There's a certain mindlessness that I see in some people as they go about acquiring new gear, almost as if they've been hypnotized or brainwashed.