Why Don't Skewed Instruments Skew the Background?

With X-Plane 9.20 you can stretch the shape of generic instruments, to create instruments that appear to be in perspective.  But why does this effect apply only to the overlays and not the burned-in backgrounds?  Two reasons:

1. Some planes are made by cutting out photographs of real cockpits.  So the source imagery may already be distorted.  The current feature distorts only the moving parts that have to be dynamically distorted, but lets you use pre-distorted imagery from a photo.
2. Our distortion might not be as nice as what can be done with high-end image editors like PhotoShop.  By pre-distorting the image you can get the best image quality.

And of course, the implicit reason 3 is that I'm lazy. ;-)