Simply stated, it's because this site is as much a social site as it is a chess site.
Why do people post so much on forums

I like helping and learning. That is why I'm so active on forums. I am VERY active on a LOT of forums. Sure, it takes a big chunk of the day, but I get my work done and enjoy my time posting and helping and learning.
:)

Why do chess players talk after the game?
Who else can we talk to about this thing that fascinates us so?

Why do you go to the movies? Why do you paint? Why do you do anything you like? This is kind of a silly question.
Why is it that people post so much on forums? I mean this is a good way of getting more knowledge but why do some people spend so much time on it.