With taxi drivers, it's often the simple desire to make ends meet. The tend not to own their taxis, but instead rent them on a day by day basis from the local Taxi Baron. So a certain substantial percentage of his day's fares must be handed over to the Baron, they usually have to pay for their own gas, etc. And since rates are usually regulated in each city, they absolutely MUST drive as fast as possible in order to get more fares and thus make enough money to actually eke out a daily profit.
This isn't a defense of the driver's major screwup, of course, just an explanation.