Actually, it does. At high zoom, you are just magnifying the JPEG artifacts and averaging errors. Such a magnified image is actually harder for the human eye to read. It confuses the human visual system, keeping it from seeing the forest for the trees, so to speak.
To see what I mean, look at this. Then step back across the room from your monitor and look again.
If you zoom into a small image of a face, expecting to count the zits, you not only can't see the zits, the face itself becomes harder to recognize. Same goes for text.
The above image by the way illustrates the effect of magnifying averaging errors. It contains no JPEG artifacts. They just compound the problem.
Well, that’s granted up to a point. But a font that’s illegible because of JPG artifacting at a high level of zoom doesn’t become suddenly more clear at a lower level of zoom, our eyes and minds simply ‘fill in the blanks’ so to speak. In this case, by doing us a favour in reducing the original size of the image down to something more managable, artifacting was introduced that obscures some of the finer details of the image, especially around the serif fonts which make up the bulk of the image.