It can't for the same reason python can't - every part was designed without it in mind.
> Python already does support it
The language runtime might have it in a branch, but the vast majority of C code it is based on, and the scripts themselves assume otherwise.
> fast single core machines
Once again, if you are using python as a scripting language, with C libraries, and spawning processes, you are already utilizing multiple cores without any adjustment on your part.
> CPU bound workloads become ever more important.
If that's true, then you shouldn't use python. It's about 100x slower than C. How much performance can you squeeze out of dividing work into cooperating threads, that can't be easily achieved by having multiple processes.
In the "web application" example you are using, the norm is already to have many processes to handle incoming connections.