The main reason that I suspect that it doesn't exist and the immediate thing that springs to mind is that carbon fibre is pretty effective at blocking RF signals. Due to the highly customisable nature of multirotors it would be almost impossible to offer a standard shell to suit everyone as we all have our own priorities on how we want certain aspects of our machine to perform. Anyone that even considers for a moment that their machine may get some kind of FPV use will not want a shell over the top of their RF sensitive electronics of any kind and least of all a carbon fibre one. Antenna placement (and I've got four different sets of antenna of one type or another) would become even more crucial than before.
There's a good reason for the pole design (although some people don't use it and just stick the GPS antenna directly onto the arms of their multirotors) in that your GPS antenna is getting signals not just from right above you, but potentially from the horizon on all sides of you. If the antenna is placed onto a surface that could block the GPS signal from below then climbing to high altitude with your multirotor could result in affecting the signal from satellites which are transmitting a GPS signal at or near the horizon. Putting the GPS antenna up and out of the way of your aircraft's airframe is highly desireable, not only because of the better GPS signal reception but also because it is then further away from other electronics which may affect the GPS signal.
Even though I love the notion of carbon fibre bling it would likely serve absolutely no practical use and installing such a thing is likely to be a complete liability.
