What you’re describing is basically using MPI in some way, shape or form. This works, but also can introduce a lot of complexity. If your program doesn’t need to communicate, then it’s easy. But that’s not the case for all programs. Especially once we’re talking about simulations and other applications running on HPC systems.

Sometimes it’s also easier to split work using multiple threads. Other programming languages let you do that and actually use multiple threads efficiently. In Python, the benefit was just too limited due to the GIL.