Why is the Book of Kells more beautiful than readable?

Because it was not mainly made to be read. The Book of Kells is a Latin Gospel book of about 340 leaves, or 680 pages, made around the year 800 and regarded as a masterwork of Western calligraphy and the pinnacle of Insular illumination. The book had a sacramental rather than educational purpose. A large, lavish Gospel like this would have been left on the high altar and removed only for the reading of the Gospel during Mass, with the reader probably reciting from memory more than reading the text.

The evidence is on the pages. There are numerous uncorrected mistakes in the text, and lines were often completed in a blank space in the line above. The makers planned twelve pages of canon tables but, for unknown reasons condensed them into ten, which rendered them unusable. In general, nothing was done to disrupt the look of the page: aesthetics were given priority over utility.

And the look is astonishing. The Chi Rho monogram has grown to consume an entire page, and hidden in its swirling decoration are animals and insects. In the 12th century, Gerald of Wales described seeing a great Gospel book at Kildare, which many have assumed was this one, and wrote that you might say all this was the work of an angel and not of a man.