I suppose you could pull it out and put some reflective foil material on there. Everything that is added to the camper add weight, and cost, so they probably wouldn't do that at the factory.
Model X19H Expandable
GMC Sierra Pick up
Been a camper since I was a kid, and that was a long time ago :)
From pup tents with no floors, to tents with floors and flies, to a pop up, to the expandable.