Most typed documents are vertical. Most screens are horizontal. Put the two together and you get a whole load of wasted vertical space on either side of the document.
Putting horizontal toolbars on the horizontal screen makes the space for viewing the document even more horizontal. And this is a bad thing. Heck, people spend extra money to buy screens that can rotate so that they can work on vertical documents on a vertical screen, hence not wasting any space. But not everyone has those.
It would be better just to align the toolbars vertically. This way, you make the viewing space for the document just that little bit more vertical, and use up a bit of that wasted space on the side:
