Another reason is that exploratory programming is hard by design in Rust. Rust is great if you already have a spec and know what needs to be done.

Most of the gamedev in my opinion is extremely exploratory and demands constant experimentation with design. C/C++ offer fluidity, a very good and mature debug toolchain, solid performance ceiling and support from other people.

It will be really hard to replace C++ in performance/simulation contexts. Security takes a backseat there.