Because it forces mastering two languages, or depending on third party developers for anything that matters beyond basic OS scripting tasks.

It became idiomatic as there was no other alternative.

You don’t have to master Rust to use this, the same way you don’t have to master C to use all of the critical extensions written in it.

(Besides, no language has this regardless of native extensions: a huge part of Python’s success comes from the fact that there isn’t a perfect graph of competencies in the community, and instead that community members provide high quality abstractions over their respective competencies.)