I meet plenty of people who'd like to do their own business, start a new career with what they geek over, and so forth. They'd really like to do this, they're quite positive.
They don't suceed or have trouble.
I know I've gone on about this before, but I wanted to add one more thing to the mix of "why fans and geeks don't have careers that make them happier" – and that's that sometimes your fandom itself can be the barrier.