A big part of this is ecosystem. You have so many game libraries and software that are written in C and C++. To many gamedevs, Rust is just a systems language Go. It doesn't bring anything significant to the table compared to just using a limited subset of C++.
The big game mills don't care about languages, they care about the bottom line. If you can churn out the same quality product with more junior engineers because the language is less difficult to master it is going to happen rather soon.
