I noticed that despite windshields being laminated with PLASTIC, they don't yellow over time (that I'm aware of) and so I was wondering, why don't they do the same for plastic headlights so that they don't lose their clarity over time? I mean one real benefit to glass headlights is that they only get dirty, and if they're going with plastic to save on weight, why not do something to make them get the same benefits that would cause other plastics NOT to yellow.

 

Windshields aren't exactly laminated with plastic. They're a 3-layer lamination, with glass on the outside, and plastic on the inside. So the plastic on the inside isn't exposed to air at all, and doesn't degrade the way plastic headlights do.

The same could be done for headlights, but it'd be expensive and pointless; it'd be easier to just use glass since broken headlight glass isn't much of a safety hazard. But there again, it'd be expensive to form glass into the shapes modern headlight lenses come in, and they'd be really heavy and thick.