I don't see why someone hasn't yet, honestly. Maybe they're worried about building technology that's dependent upon a sole chip vendor like NVidia, but in concept its not too different than something like powercore, only way more advantageous for them since NVidia GPUs are already so prevalent. A lot of people have them and don't even realize it.
The rig I'm typing this on right now has a gtx690 with 4GB of memory on the card! That's more than a lot of folks laptops have for primary RAM!
Talk about an under-utilized hardware resource. Nuts that there isn't more audio stuff available for it.
On the other hand, we could be using our GPUs to mine Bitcoins, ultimately putting cash in our pocket for frivolous expenditures like overpriced synth hardware