The main reason is that a lot of scientific computing depends on ancient libraries written in Fortran or C, and a lot of newer libraries depend on GPUs for their matrix operations so need to be able to make calls to C/C++ modules that run directly to the GPU. This probably isn’t impossible for Java but that was never meant to be it’s niche, it doesn’t compile to machine instructions and prefers portability.
I know FFI in Java is certainly possible and I’ve seen it at least a few times personally; is it that much harder than in Python or just less common due to the need?
Python doesn’t compile to machine instructions either and there’s nothing that prevents GPU access from Java. In fact I’d bet in many cases pure Java beats Python + C library though it obviously depends on how much has to be written in pure python.
