As explained in this TED talk by Dan Pink, extrinsic motivation is a great way to get people to do boring and repetitive tasks, but it actually harms performance on more creative tasks. Creativity is a surprisingly fragile thing. It comes from deep inside, and external concerns (most especially, "What will people think?") seem to scare it away. But that's a topic for another time.

I don't care how much you pay me, you'll never be able to recreate the incredibly satisfying feeling I get when demonstrating mastery within my community of peers. That's what we do on Stack Overflow: have fun, while making the internet one infinitesimally tiny bit better every day.