A lot of complex GUIs are written in C++ (or are thinish wrappers around an underlying toolkit that is C++). This is often for performabce and/or resource consumption reasons. UIs may not have hard realtime requirements, but they are expected to consistently run smoothly at 60fps+. And dealong with multiple screen sizes, vector graphics, univode text,r etc can involve a lot of computation.

Rust gives you the same performance as C++ with much nicer language to work with.

Used to be written in C++, and usually trace back to the 1990's when C++ GUI frameworks used to rule.

Nowadays most are written in managed languages, and only hot paths are written in C++.

There is hardly anyone still writing new GUI applications on macOS, Windows in pure C++, even Qt nowadays pushes for a mix of QML, Python and C++.