Why is the 12 volt electrical system used in car's?

You are all wrong. It is 13.8V and not 12V.

A car battery is really six lead-acid batteries connected in series. Because of this complex construction, the weight is greater than if a similar quantity of lead and acid were used to make a single-cell battery of two volts.

The benefit of more cells is that every time you double the voltage that is driving any electrical device, then for the same power output, you can reduce the amount of current flowing through the device to a quarter of what it was before.

Fifty volts d.c is the highest voltage that doesn't need special insulation or precautions to avoid electric shock but even twelve volts is enough to reduce the current through a car's starter motor to a manageable amount of only a few hundred amperes.

This makes a good choice of standard battery size that is neither too heavy or requires too many cells.

12 v for normal cars is an optimum. Enough to power starter motor, decent lights etc but not so much as to risk sparks and even fires. The higher the voltage the more serious has to be the insulation bearing in mind that cars travel around in wet weather. New electric and hybrid motors do operate at higher voltages but they retain a 12v service voltage for same reasons.

If you can get a car to run on 12V what would be the point in making the V higher, 12 V is a very safe limit and your not going to die touching it are you. Just think how many people shock themselves every year on the battery and even changing a light blub. If it's not brokern - why fix it.

The battery would need to be larger and take up more space. The alternator would need to be quite a bit larger and make the siting of it difficult.

I think 1 reason would be that it would require a battery that is physically larger.

So that you can use the same batteries for Trucks, Tractors, Boats, Airplanes, Rv's

Watts = I^2R V = IR 12 V = (0.22 A)*R R = 12/0.22 = 55 ohms Power = (.22)^2 * R = 2.6 watts

it is mostly because of the safety features