We place a negative cable to the ground instead of the negative terminal to prevent an explosion. Avoiding sparks near the battery is the main reason for the common advice to make the last connection to ground away from the battery on the vehicle with the dead battery.
The negative battery terminal isn’t grounded because it is attached directly to the car’s chassis. Many cars have an electrical system that uses negative ground, and grounding would cause a short circuit, making the situation even more dangerous than just having ungrounded electronics.
However, anything not connected to the earth (such as a battery powered device, or a vehicle) still employs a common node commonly called a ground. When you use the negative terminal of the battery as a "ground" it is really just the common node (return) of the circuit.
When you use the negative terminal of the battery as a "ground" it is really just the common node (return) of the circuit. A circuit has to have a closed loop for current to flow, and using the negative terminal ensures that all of the components have an identical reference point.
@jrista: Yes, "ground" is usually the negative terminal of the power supply. In many circuits, you will see ground symbols scattered around the drawing - these should all be connected together. Using ground symbols like that is intended to reduce congestion in the drawing.
We tend not to do it nowadays because negative grounding reduces galvanic corrosion of the frame of vehicles. You can call anything in a circuit "ground". "Ground" is just a reference point. Ground can be anywhere in the circuit, but it is usually near the power supply, because current going into or coming out of ground needs to go somewhere.