Plastic Vs. Glass
UV light & oxygen (particularly in combination) affects plastic very differently than they do glass.
Also, on more modern cars, there is a very thin coating over the lens, and this an even more susceptible plastic.
Glass absorbs uV, but it's not an organic (carbon containing) material, so the light is merely absorbed & converted to heat.
They could use glass for the headlight lens, but car makers look for every place to reduce unnecessary weight. A pound here, 5# there & the car weighs 2% less which improves mileage. So, plastic headlights are here to stay.
Because of the obvious safety risk of a foggy or distorted, or prone to glare windshield, that's not something they can replace with plastic. So, it stays glass specifically because it won't fog over any reasonable lifetime of a car.