[jboot]

Police are not trained to shoot the tires, and they never have been, except in the movies. Shooting the engine block with a service pistol is likewise a potentially very expensive gamble.

To stop a vehicle, police are trained to shoot the driver.

[Cvengr]

Tires are sometimes very difficult to 'shoot out'. Bullets can bounce off of them.

[SoVaDPJ]

I don't know much about cars or shooting at them, but...if you shoot the driver, doesn't the car then go out of control? Again, I don't really know anything about it, but it seems to me that controlling the car might be the first course of action.