Why don’t you see words flipped in a mirror?
Why should the mirror care, whether it is left right or up down, you might want to ask. You are looking for visual things to happen the same sideways and up and down. Why not? Gravity, the pull towards the center of the earth, is what makes up and down different from side to side, but why should that affect something visual.
The surprising answer is that, in a way very consistent with words appearing “backwards,” you DO see words flipped. In other words, the mirror doesn’t care.
To explain my answer, let us first consider your image in the mirror rather than the reflection of words, though the descriptions apply to both. A very natural way to check if your image in the mirror is “true,” is to imagine yourself rotating 180 degrees while standing on your feet, and looking back out from the mirror. Compared to that imagined picture, what you see in the mirror is transposed left to right, and is what is well-known as your mirror image.
Now, another very natural way to check if the image is true is to imagine yourself rotating 180 degrees by bending over forward to stand on your head, and looking back out from the mirror. Compared to that imagined picture, what you see in the mirror is transposed top to bottom (that is, you see yourself “right side up”). It is again a “mirror” image.
And also for words, they are indeed a mirror image vertically. As well as being one horizontally. The former (vertically) leaves them looking like what we expect to see (not flipped), while the latter (horizontally) leaves them in an unusual form, (“backwards”) or what we call their “mirror” image.