Its the plastic that they are made from, all plastic degrades over time in sunlight. Some cars use cheaper plastic and they suffer the worst. If they had the film applied over them would that help at all?
My Xfire spends most of the time in the garage but it still gets the hazing, not much as I can rub it off with my finger. Sitting in the sun in Arizona must be a ***** (sorry, she dog)

Edit, I used the b#tch word and it was edited, is that a swear word? What can I call my dog, a she dog?