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

Usually operating systems are pretty simple, if you look at their structure. But parallelizing them will not improve speeds much, only raw clock speed will do.

GPU's simply lack parts and a lot of instructions from their instruction sets that an OS needs, it's a matter of sophistication. Just think of the virtualization features (Intel VT-x or AMD's AMD-v).

GPU cores are like dumb ants, whereas a CPU is like a complex human, so to speak. Both have different energy consumption because of this and produce very different amounts of heat.

See this extensive superuser answer here on more info.