For 1080p screens, on the vertical you only have 1080 pixels which is pitiful once you subtract 300px worth of start menu, title bars, menu bars, ribbons, status bars, url bars. Even with higher resolution it looks like a letterbox.
Additionally, like you said, for text, you ideally want windows with portrait aspect ratios - but if you split a fullHD 16:9 screen in two, your portrait windows only have 960 pixels of horizontal resolution which is not enough for many IDEs (or e-mail clients or spreadsheets).
On a 28 inch 4K screen 16:9 is not a problem - you can have 3 long vertical text windows next to each other, or ultra-wide spreadsheets, or a combination thereof. On a 13 inch laptop screen it is a different story.
