That's because concurrency is really in the SDK or stdlib for the language or platform.

In my opinion, that's really the best place for it. From what I understand, that's always been FORTRAN's secret sauce.

I've been doing threaded code since Day One (Machine Code), and have gotten it wrong, quite often. There are a million things that can cause concurrent code to actually perform worse than single-thread code.

These days, I usually just let the platform pick the threads, and generally write code that will play along.