The question hits on some of the most fundamental aspects of our current understanding of reality and theoretical physics. As another commenter pointed out, one potential answer delves into QFT. Just because OP used a metaphor doesn’t warrant you saying they had “too many pot brownies” and there’s absolutely no need to be a condescending jerk here.
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.