Mostly because Python is a great "glue" language. It isn't performant enough to implement the actual low-level computation, but is better at running other applications, getting data from them, feeding them into other applications (a.k.a pipelines).
However, if you're more algorithmically focused, python is a great DSL
The rest of the ML world is in that exact situation, but on Python. They aren't going to throw away their familiar tools unless everyone else does too.