Browsers have quite complex interactions with operating system runtimes and this is largely a huge pain in Rust, especially on MacOS. Binding crates are outdated, incomplete and you’d spend lots of resources writing (unsafe, crash-prone) wrappers for a shifting target which is work you wouldn’t have to do at all if you used (objective) C++.

Software engineering is more than writing a program, there are economic factors at play too.

That being said, I also wish my browser was written in Rust (and my OS too!).