Just because traffic is eternally at a standstill in New York doesn't mean taxi drivers won't find a way to gun it from zero to ludicrous in under three seconds. The reason most citizens can't hail a cab is because from the curb all you can see are yellow blurs of light hurtling F-bombs warped by the Doppler effect.
Supposedly cabbies drive the way they do to quickly get more customers, and thus earn more money. It's more likely that they're all secret participants in a no holds barred Death Race, picking up innocent passengers along the way for cheap thrills and deranged side bets. This would best explain why, despite constantly screaming at carpoolers, police officers, and bike messengers, cab drivers save their most burning hatred for other cab drivers.