Just to stir the subject:

Most plastic headlamp lenses are made out of Polycarbonate (PC such as
Lexan and other commercial names).

PC is used because it is a tough material, does not break easily, and
withstands high temperatures, but unfortunately, it is one of the worst
materials to use outdoors because it is highly susceptible to UV
radiation, even when loaded with UV stabilization additives.  The UV
radiation from the sun is what turns the clear lenses into yellowish
light intensity hogs.

So why is it used?  Weight, Cost and moldability to conform to weird
designs.  Besides, manufacturers do not mind the extra business of
selling replacement lenses at outrageously high prices, do they?

As an after-thought:  PC is derived from oil, while glass, for the old
fashioned headlights, made from sand.