If you hunt around for the debugging working group and related issues there's some interesting reading on exactly that. (I'm on my phone and it's been a while since I looked into this). The issue is that a debugger generally can't assume your code doesn't have bugs.

This would make bugs around state in Debug impls much more complicated, for example. This can creep in under a few layers of indirection - ultimately calling a function that gets the value of an interned string would be one example.