It also disregards the fact that there's a pretty significant leap between coming up with something new to do with an already-existing character and writing a completely original work. I've actually gotten ideas for original works but I haven't committed any to paper for a really long time (let alone tried getting them out into the world) simply because I can create a character, but not a world. Or I have a world, but not a plot. And so on. Whereas with fanfiction you have a world already established, and characters, and your biggest challenge is writing the story you want to write while still keeping the characters in-character.