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.

The only software that is written without concurrency in mind is software that uses very few resources.

Anything else heavily uses concurrency in all kinds of ways.