Why? C'mon guys, we all know there is no why!
Ok, I'll bite
Just read through the other replies and they make this a very interesting thread altogether.
Back in 2005 I joined TheJawa, which had a very solid customizing section. Back then I didn't even know such a thing existed! But Hasbro's tardiness in releasing a Galactic Marine made me do some research on the matter. I started lurking the customizing section there and read all of the tutorials on FFURG. In spring 2006 I took a small leap, bought some Citadel paints and made my own Galactic Marine.
So I customized it because Hasbro hadn't made one yet.
However, as you all know, customizing doesn't stop with one figure. As was also the case for me. I tried to improve on my Marine design, but when Hasbro finally released their Marine (which, apart from the silly feet, is an awesome trooper) I shifted my focus on other characters. Comic book characters and fan fic ones. And now I don't customize something, because it hasn't been produced yet, but just for the fun of it.
And it has gotten kinda out of hand I must admit. From one single Galactic Marine, my new hobby practically went to
warp 9 hyperspeed to encompass lots of WIP figures, sculpting, dioramas, vehicles and contests!
I have sold a few of my early customs on ebay for a couple of bucks, but I'm not running a business. I'm too slow for that, and time's always an issue. Occasionally I make a commissioned figure for a fellow board member, but that's about it.