An old question, for sure, but have you really thought about it? Fame, fortune, both? Control of the world in some way? Ego? Leaving your mark?
Most of us began writing, or considered it, long before we knew there might be fame or fortune in it, so it has to be something more intrinsic. I'm convinced I do it to somehow leave my mark on the cave wall, because I don't want to be totally forgotten. I'm one of those people who want to live forever and can not understand how some accept their mortality. If I could live forever, I'd do it. The risk would be worth it, and you can always check out if you're not satisfied. And the time is coming when we will live indefinitely. Well, maybe not us, but our great grandchildren. But as important, if we knew we would live for a thousand years or more, would we still want to write, or be as passionate about it, as desperate to get words to paper? In a world of indefinite longevity social needs might be radically altered.