I used to play an MMO where one friend of mine must have spent more gold than I ever owned on changing his appearance, because I don't think he looked the same for two hours straight. And then there's me, all my toons always look the same, and in every new game I play, I make them look like that too. It's the same with forum avatars: I never change mine, except when I put a Santa hat on it for Christmas. I've had, um, three in all the time I've been here -- the default newbie one, the one I had before this one, and this one -- plus the Christmas special. It's all about what people feel like. Some like to swap around their avatars all the time, some never do, whatever is fun for them.
Pretty sure because people like me just feel like it, as Jefe mentioned. On a (though now inactive) forum I am an admin at which i joined in September 2007 I have changed my avatar apparently 351 times. Time sure flies.
I doubt I will ever change my avatar. I like it, and I hate it when other change their avatars. I recognize them by their avatar, so when they change it, it just makes it harder for me to easily see what posts I want to read. Despite this, it isn't my decision, and I don't really know why some people change it so much.
I change my avatar every now and then, often when I'm in the mood for something else.
Ironically, I hate changing my avatar as well, because that means I have to change my signature to something that matches, which can mean having to make a whole new one from scratch. You would not be able to imagine how many hours I've spent on making avatars and signatures for this forum. I envy Badprenup his ability to stay default, not having to make pointless images to feel good about himself.