Seemed odd to me as well until I saw a live bug inside the light moving here n there. I set off to detect the opening and found out that the lights can very easily be removed by unscrewing two bolts at the back. When they're off, you'll see a screw at the back, unscrew that and you'll have the reflector housing in your hands, no seals no nothing.
That's where the dust goes in and fades them. Thankfully, it was a bit early for my car so chrome wasn't damaged at all, I cleaned it thoroughly and applied Interior care Protectant on them for seam less cleanliness and shine!
An easy task for a DIY person like you (and me :P)
