[Panelfantastic]

I don't know the tech reasons why OEM wouldn't do it but I'm willing to throw out some ideas about retro fitting a set.
I bet racers don't do it to keep the heat out in the airflow and to keep the clamping force close to the largest area of rotatating mass and for design/packaging reasons (they usually run big rotors and calipers).
The only place you see it in the aftermarket are street rods where cost is usually not a concern, you would have to have some pricey billet pieces made and you would have to design everything yourself.