Probably because people get taught how to use low level primitives like mutexes and then proceed to have shared mutable state (aka two threads write to one memory location). For the vast majority of applications the complexity of shared mutable state isn't worth it. They should instead restrict themselves to only having one thread per memory location that is allowed to write to it.

For 99% of applications that means you take your favourite threadsafe queue implementation for your specific language and launch threads and only communicate this way. The messages should preferably be immutable, copies or at the very least marked readonly via const if copying is too expensive.

I'd recommend you to take a look at Erlang.

Also remember that there is no magic pixie dust. Functional programming languages are not inherently better than non functional programming languages at concurrency and parallelism. They merely leverage the "no multiple writer" principle by default. Immutable data is threadsafe primarily because it's guaranteed that there is only a single writer.