TypeScript is way, way too slow to use for desktop applications. People try and, surprise surprise, those applications are slow as shit.

Qt has a C++ backend, so C++ is used. Qt is relatively safe and well designed.

TypeScript transpile to Javascript - do you really want your operating system's shell written in slow ass Javascript? Modern C++ with hardening rules for the compiler[1] is pretty safe.

[1] https://best.openssf.org/Compiler-Hardening-Guides/Compiler-...