cosmic: Then it would be too hard to repair. You would have to replace the entire thing even in case of a small dent.
In theory yes.
In practice there will be some problems. Certain parts of a car body need to be VERY strong to hold the suspension, engine and protect the passengers. Other areas can be lighter. So do you make it out of plastic, and it breaks up 6 months of driving? Or do you make it from titanium metal, which would be incredibly strong, and even more incredibly expensive.
There is also the issues of repairing it. Get a guard dinged and you have to buy a new body for your car?
So I'll go with, technically yes, but practically, no. Making individual PARTS that can then be bolted / glued together with other parts, sure.
It's possible.. it's done with fiberglass and carbon fiber all the time. The problems though is damage.. one dent or ding may cost an entire body. As far as safety the crumple zones are in the frame not the body in most cases.
It could. But it wouldn't be strong enough.
Sure. But, it would not be street legal. It would crumple too easily.