That's very interesting but why do taxi drivers have a job that lowers life expectancy? Is it because they're sitting too much? Or because they're inhaling CO from cars too much? Or staying inside (the car) too much? Or is it just stressful?

For from the look of it it doesn't look like an exceptionally hard job (btw a brother of mine did years of "VTC": "Vehicule de Tourisme avec Chauffeur", basically high-end taxis without the "taxi" logo).

Because it doesn't pay very well. Being affluent increases life expectancy.

Getting stabbed in the neck by filthy street urchins.