I'd answer it the same way I'd answer "why do you write so many applications". I have a need to constantly keep my skills up to date, and fresh in my mind.
Unlike most other jobs, computer-related studies tend to overwhelm the mind, and it's a lot easier to forget the specifics of things if you don't do them regularly. As I'm learning things, I tend to do them more. For instance, I recently started to learn C. I have 190 different programs that I've written in C, the vast majority useless apps that were merely created to make sure I understood the functionality of something I've learned. I have probably four or five actually useful applications. For me, it's a desire to know that I know what I'm talking about, to make sure I don't let myself slip.
Personally, it's not so much because the mountain is there, because I know I can climb it. A lot of the time, it's to prove to myself that I can still climb it.
