It's also a cost thing (and heat/reliability/etc). Could simply be that the CPU is more complicated and so has larger gains from a reduction.
These are extremes, but if one part costs $200, and another part costs $1, youwwant to put your resources in reducing the cost of the $200 part not the $1 part.
(The ratios are no where near that, but I am sure the CPU costs more than the GPU).








