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An optimizing compiler may not be better than me at optimizing hot code paths, but my time is a very limited resource. The compiled version may only be 75% as fast as my hand optimized version, but writing that hand optimized version will likely take several times longer. Sometimes it is worth spending the extra time for that performance, but usually it is not.

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Indeed, plus you need multiple hand-optimized versions. Not only per architecture, but also e.g. pre-AVX and AVX. An optimizing compiler will give you optimizations for all current and future platforms for free.

Another problem is that the number of people who can write good general hand-optimized assembly is small. E.g. I used a numeric Go library (which I will not name, because I should've submitted an issue) that used assembly-'optimized' routines. Replacing those with simple C loops and turning on auto-vectorization beat those hand-written routines handsomely.

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