Because once you've committed to a fixed fee, every capitalist bone in your body is motivated to get stuff done fast. Of course, fixed fees do carry the risk that you'll underestimate the time and resources required for a project, and make less money than if you'd charged by the hour. Your administrative overhead is minimal, you've only got one client relationship to manage, and you may even gain use of the client's spiffy facilities. Perhaps the biggest disadvantage is losing the diversification of income sources that makes an independent professional feel independent. You'll face the ever-uncertain future with the fearful attitude of an enthralled employee, rather than with the feisty independence of a free agent.
Free agency is risky, but employment is riskier.
As an employee, you're putting too many eggs into one fragile basket. If you don't take the leap, you may never find out just how well you could do as an independent. That's a risk that millions of us free agents just can't bear.