Speaking of cabs in Beijing: how come I can never get one? I see empty cabs drive by constantly (and not just foreigners, locals too). Or they sit around not

Speaking of cabs in Beijing: how come I can never get one? I see empty cabs drive by constantly (and not just foreigners, locals too). Or they sit around not wanting to pickup fares. The whole cab system seemed broken when they didn't want to drive people places. Anyone have any idea what sort of incentive system these guys have or try and explain this behavior?