There have been bugs that allowed you to do crazy things because of Safari’s JIT before (to the point they added a switch to the OS to disable it). Maybe they figure if Apple can’t write a safe one nobody else can either?
I think most of us on HN understand that "safety" is something you just have to accept is never 100%.
As an example a team responsible for security can test the standard image libraries (libjpeg, etc.) using file-fuzzing to find any bugs around buffer overflows, etc. It's good they do this but we all know that this is still a statistical shot in the dark that may still leave a vector somewhere in the framework.
https://support.apple.com/en-us/105120
So Apple considers its own trusted JIT a security threat surface.