Because the desktops are viewed from such a greater distance, the are effectively retina displays anyway. Also, the cost of making retina displays that large is going to get pricy, and yields are probably lower the larger you make the display. There probably are problems with uniformity, dead pixels, etc, when you attempt to make the larger. In time, it will probably be possible to get higher res screens, but apparently not today. Apple only delivers new tech then it is feasible to do so at their price points and in the volumes they hope to move, as you know.
