Why not replace the GPU CPU?

Parallel processing is a powerful gpu, cpu gpu are not a lot of features. What instruction pipeline, multi-process management and the like. gpu do not have much capacity for independent processing instructions, basic instructions on cpu is calculated by gpu. GPU works cpu processing instructions, gpu meet local needs, such as matrix processing, image rendering, will open up a small space in the memory, and then put this data to the much smaller matrix is ​​labeled pipeline to process gpu, gpu behavior basically rely on the mobilization of the cpu, the instruction itself is not strong enough (except data processing).