because it rust easier .!!
Also:
http://en.wikipedia.org/wiki/Honda_RA302
This is what happened when they tried.
Magnesium is a very flammable metal when heated to its flash point. Once it starts to burn, it is nearly impossible to extinguish, hence its use in magnesium road flares. It can be safely used in some alloys, but the cost is prohibitive.
Too expensive
galvanized steel and plastic
The ferrous metal commonly used to make car bodies is steel. Steel is chosen for its strength, durability, and formability, making it ideal for providing structure and protection in automotive applications.
tempering process n casting process
Magnesium can be used to make things like alloys, such as aluminum-magnesium alloys that are lightweight and strong. It is also used in flares, fireworks, and sparklers due to its bright white light when burned. Magnesium oxide is commonly used in supplements and medicines.
Because it is expensive and a fire hazard. Also not easy to work with. So the advantage of low weight is outbalanced by the disadvantage of price and complexity of machining.
While Magnesium is very light - Cars would be very expensive to make out of Magnesium plus Magnesium is not structurally strong enough to withstand a vehicle collision and - not to be ignored - Magnesium is a metal thst BURNS....and burns very hot - not good if your car caught fire.....Very Unsafe.
Iron
Because Georgina Sentinella Said So !
Galvinised mild steel,
Usually, Alloys have better properties - like strength-to -weight ratio - than the parent metals. Pure Magnesium is also a bit nasty to use and work with as it can Catch fire.