Do deserts have ravines, that are open to the surface? I have seen some surface ravines directly at the border of deserts, but never in the middle of a desert.
In world generation logic, desert sand placement comes after digging out ravines. Since deserts come with a solid layer of sandstone underneath them, you will never find an above-ground ravine in a desert.
I'd imagine that this was intentional; realistically you wouldn't ever find above-ground ravines in deserts because they'd simply fill up with sand.