well... as Rouxenator said, gpu's are inaccurate... they good at assembling geometric shapes, and at placing pixels, but the way they process is somewhat all over the show, you might end up having lost bits of information and such... but gpu's are designed to do precision processing, they aren't designed at calculation proteins, rather, they designed for graphics. and now companies are seeing that gpu's are fast enough to do some general processing. So, when we start to find physics in games become very complex, errors in the processing will become present...
this is why intel is pushing ray-tracing on the cpu, as you might find errors, anomalies and miss calculations with the gpu, and, yes, ray-tracing can be done of the gpu, but for it to be accurate, the gpu will need to double check everything, which means a mega performance hit...
as for the GPU on a CPU post,
you must remember that once AMD incorporates these two processors on one ship, it would mean that the performance increase each year would be greater as they can have increased cpu and gpu performance and people would feel obliged to upgrade more often... Also, if they pull this move, more people would want to buy an AMD cpu/gpu as it's cheaper and possibly better performing then intel...