A creator with sufficient industry clout could negotiate a deal where they receive royalties, I imagine. But you're right that most aren't in a position to do that. So they do the next best thing, which is build up a name for themselves using Marvel's and/or DC's existing content until they have a sufficient fan following to do their own thing. Most writers and artists aren't really all that successful going it alone, but one good graphic novel with your own story and creations can make you quite successful, especially if Hollywood picks it up and churns out movie adaptations based on your book/graphic novel.
All that said, if the choice was mine, I'd create an original character for Marvel in a heartbeat. But that's the fan in me talking, not the businessman.