I'm not a bike courier, but I would think the answer is obvious: overhead. No gas, no car payments, no insurance. And if you're riding within the downtown core, depending on the time of day, it's probably as fast or faster than driving a car more often than not.

But you're right, it's obviously extremely dangerous. I drive my bike year-round to get to work and I have had my close calls - and that's just 50 minutes of biking time a day. If you were doing it non stop, let alone at top speed, it would not even be a question of if but when you end up in the hospital. Russian roulette.