Saturn used composites and plastics for their bodies. Repair was extraordinarily expensive and had to be done by specialists. There were other problems... see the source.

It's much harder to mass produce composites compared to stamped steel or injection moulded plastic.

You can have a machine spit out hundreds of pieces an hour. Or you can have skilled workers laying down layers of composite on moulds and letting them cure.

So it gets reserved for exotic sports cars, jet fighters. multi million dollar airliners etc.

they are used. composites are used on intake manifolds, valve covers and oil pans for starters. As well as some body parts. Every year they use more and more this is how they lower bodyweight of the car. FYI some things you think are plastic are actually composites.

They are used. Citroen used them as far back as the BX for panels and the whole tailgate. Other makers use them in places. It is a cost v weight thing.

Costs. The more expensive the materials, the lower the profits.

They still have not come up with a manufacturing method that cost efficient. It will come soon maybe in 8 years.