Swift and Kotlin are used for application programming there, not systems programming. Also, Google does Rust as well. And Microsoft is slowly switching some pieces to Rust as well. These are big companies with many teams and use many different tools.

Don't know how iOS, but Android does offer native APIs, so Kotlin is not the only officially supported choice there. Actually for anything that needs performance, e.g. games, Kotlin is a nogo.

Swift is used for systems programming on Apple platforms, as already replied on another thread.

Google already mentioned a couple of times at Android Fireside Q&A sessions that it is weighting Kotlin/Native adoption on Android.

And even if not, Rust isn't taking C++ and Java place on Android, specially after Project Treble changes, where drivers can even now be written in Java.

Google and Microsoft are indeed adopting Rust, with some products already in production, although they are also among the major ISO C++ contributors, so it remains to be seen how much Rust love from their security teams will spread into the OS development teams.

For example, I still look forward to the day that Azure Sphere actually offers something else other than C, in spite of its security sales speech.