Polycarbonate plastic is used because of its tensile strength & the ability to be manufactured to conform to the front end styling of a vehicle.

Although it's a very hard plastic, it's also very porous so during manufacturing a protective silicone hard coat is applied to the headlight lens.

This silicone hard coat is supposed to protect the headlight lens from intense sun, heat, road chemicals, bugs, humidity and acid rain, etc. 

But these elements, combined with the heat of the halogen bulb damages the protective hard coat to the point that a ugly discolored, cloudy, hazy appearance develops on car headlight plastic lenses.

This damage will restrict the headlights ability to project its beam of light at night, and during adverse weather conditions, may diminish the ability to see by as much as 70%.