Because even trivial things like Fourier transforms (to people like me) are very difficult to understand to those that don’t know them. They took me years to understand. Non scientific software engineers do not understand those. It’s just a different course of education.
You’re also right about old code base as well. Algorithms like these belong in c++ (or C or fortran), and it’s extremely difficult to explain why to people who have no understanding of numerical computing.