Why not using GPUs as a CPU?

Question

I know the question is only partially programming-related because the answer I would like to get is originally from these two questions:

Why are CPU cores number so low

Answer 1

Because nobody will spend money and time on this. Except for some enthusiasts like that one: http://gerigeri.uw.hu/DawnOS/history.html Dawn now works on GPU-s: with a new OpenCL capable emulator, Dawn now boots and works on Graphics Cards, GPU-s and IGP-s (with OpenCL 1.0). Dawn is the first and only operating system to boot and work fully on a graphics chip.