Why are headlights glass and taillights plastic?

Headlights are much hotter, and would eventually melt the plastic.

What @filmfann said and also rocks and other projectiles fly at the front of the car from the tires of other vehicles.

There are some cars with plastic on the front too. But the main reason of the glass on the front, is because the small sand and stone pieces during high speed would scratch the surface of the plastic in much shorter time than the glass. Pluss of course it’s easier to clean off the bugs from it too.

My car has plastic headlights.

Mostly because plastic scratches and those scratches diffuse/defocus the light. A tail light just has to glow, but a headlight needs to project a beam. My old Saturn had plastic fenders, and the headlight reflector the part that held the bulb was plastic (silverized on the inside, of course), but it still had glass lenses on the headlights.

@Nullo That is a new one on me. What type of car do you have?

@jerv More accurately, it’s the car that I used to drive, a Dodge Neon. I don’t know what my Corolla has.
I had assumed, at least, that they were plastic because of the way that the material had gotten cloudy.

@Nullo Drive enough miles on dusty roads and you’ll eventually sandblast the glass. The (definitely glass) headlight bulb I just replaced was a little frosted, and my windshield shows it’s age with over 25 years of pitting and scratches.

Plastic is used in many cars headlights now adays actually.

Glass is still used as well though.

Largely its a quality/cost thing.

Significantly less mass.

The outboard corners of a car are some of the worst places to increase weight.

@jerv The bits of the headlights that I can easily tap are plastic.