For the same reason people level Alt's in ever other MMO in existence. Personally, I get bored playing the same class for longer then a few hours, so I often weill level multiple classes at the same time. In addition to crafting classes, and gathering classes...

You sound much more like a lone wolf. One class unless necessary. Which is fine, but it's also not the only way to play the game. It's part of what makes an MMO cool, and why I'll often only play MMO's.