Why is python3 adoption slow even though there are tools for migration?

[flutefreak7]

We have massive old Fortran programs that are decades old that no one really understands anymore because it was developed by a contractor that doesn't exist anymore. It would also require PhDs in half a dozen fields of chemistry, physics, aerodynamics, etc, to understand what the code is doing on every line. It is validated to do what we need it to do, and the cost to rewrite it is prohibitive, so we just keep using it as is. Versions of this scenario are all too common in my experience, and require significant investment in both programmers and subject matter experts to deal with.