It's not only in road cycling. Look everywhere - car enthusiasts, photography geeks, computer gurus etc etc. Everyone trying to make things more, better, faster, lighter prettier.
I am one of them. Why? I am not sure. Maybe, because life is so freaking short and we are trying to get the MOST from it? We've become very materialistic and always eager for more things. The whole economy based on that.
I've asked one guy at he LBS, one of the customers. "Why are doing this to our bikes? Counting grams and milligrams while carrying around extra 20 pounds of fat in our belly? The answer was - loosing weight os no fun, upgrading parts and components IS.