Because the words wouldn't start at the same height. The point was that, if every title starts at the same height it's easier to skim, because you don't have to move your eyes much when looking at the first few letters.

So make them all start at the same height, at the top of the spine? What exactly about top-to-bottom orientation do you think precludes proper justification? You're still on about something orthogonal, the justification along the spine, which isn't affected by the overall orientation.

Also, someone posted photos of French bookshelves showing that their centers tend to be centered like ours are, so it's not any more readable. If you think of the spines as advertisements on bookstore shelves then this makes sense.