The thing is, optimizing compilers don't actually do any optimization in the classical sense. They don't do run-measure-modify loop as programmers do so all they can apply to the code is the heuristics some other programmers taught them. Some things are trivial like loop unrolling or function inlining. Some are not, like trigonometric approximations.
In the long run, when a compiler stops, a person goes further. That's why empirically, hand-made code is still faster than the compiled one. Of course you can write slow code in Assembly, but if you already decided to go low, you'll optimize your thing until satisfied with the result.
One particular thing comes to mind, C is ridiculously bad with automatic superscalarization (try VTune's HPC characterization on any code you like and see). Sure, you can use intrinsics and implement things manually, but that would be essentially the same as writing in assembly.
There is another contender though. Spiral (https://spiral.net/) that actually do solve the optimization problem can in fact be as effective as a human programmer and at a fraction of the cost. Spiral is overly specialized, so it's not a threat to all the HPC, but a more generic thing may indeed become a gamechanger.