I think it's more the humans that aren't up to the task, or at least have not built enough conceptual infrastructure to make it feasible to program these things.

A moderate number of people can grasp the imperative "do this then that" model, which CPUs strive so hard to maintain the appearance of despite speculative and out-of-order execution. Fewer people are comfortable with "functional" style programming. Other paradigms are relegated to corners. Multithreaded programming is still a bit of a black art.

Graphics programming only manages parallelism because there's an implicit huge map of "do this to every pixel" around everything. The actual shader programming language .. is imperative, because that's what people understand.