Also, to emphasize, when a cheap aluminum, steel or fiberglass part breaks/is wrecked, it's cheaper to replace than a carbon infused panel. Try to re-bend the frame of a steel car (rather easy) and re-bend the frame of a car with a monocoque body made of carbon fiber (You can't).
How a car is designed is not solely about the efficiency of it's drive train, but rather there are a large number of constraints on the automobile. How large is it? How many passengers and cargo can fit? How comfortable is it? Does it have all the nice features that modern consumers want? Does it have more than one cup holder (BMW is notorious for failing this test, an interesting cultural difference between US and German automakers) How does it do in safety tests? How does it look? Etc. etc. And again, one of the most important: How much does it cost?