Spider-Man, Batman, whoever, they all have to be somewhat relevant to the modern audience in order to sell. That means the dial gets slid back on characters' ages, their military services, their time in college, etc., and that the storytelling engine propelling them gets a tune-up every ten years or so. The pop psychology of the day gets added to the painful origin story, the pop science gets threaded into the super-power. It's the way of the beast.
A lot of writers probably see the retcon as a way of keeping characters fresh, or to make them palatable to the target audience. For others, a good retcon gives them the opportunity to tell the story they want (or are contracted) to tell. That said, I have to wonder how many times a concept can bet retcon'd before the original substance is completely lost in the shuffle...
