Think about having to develop a platic for that - hot oil in constant contact with your candidates, all day long. Hot exhause gases constantly blowing over your plastic. And since these are both along the bottom of the car, there is a large temperature differential (especially in Northern winters) that is trying to warp the parts more than dilithium crystals twisted space for the USS Enterprise.
As I've mentioned before, plastics are going to continue to be needed in automobiles in order to increase mileage. I'm just glad someone else is working on these developments.