Trouble is that in an application the pure functions are just implementation details that don't need to be tested. They get tested by virtue of a correct implementation being necessary to correctly run the application. A library can be a little different story, as there the public interface is quite likely to be pure, but I expect most codebases found in the wild are applications. The discussion happening here seems to be directed at applications.

Granted, it appears Rust gets a lot of use as a cross-language library provider, where the application glue is written in other languages. Perhaps that's why?