The volatiles "burn off" of the plastic trim ... aka oxidize.
They sell things to bring them back to life (paint) or you can put some old ATF on them.
I put old ATF on my Cherokee's plastic trim, lasts a month or so then it turns gray again. I have so much old ATF laying around that I'll never run out.
2011 Focus SE 5 Speed 36k - Blue Flame Metallic - Added Rear Swaybar
2000 Jeep Cherokee Sport 115k - Lifted with mud tires.