My assumption would be that it’s because we don’t really look at mirrors per se but rather the reflection in them, so the definite article is indicating the fungibility of the mirror itself. This total speculation on my part though and I might be totally wrong.
I mean, technically there’s nothing preventing that, but in practice it’s a fairly uncommon mistake to make and it’s immediately obvious that there’s an issue the first time that path is taken. If something like that makes it to production, it clearly points to an issue with test coverage rather than code paradigm.