Almost all business applications have horizontal menus and ribbons that take up a decent percentage of a landscape monitor instead of utilising the “spare” screen space on the left or right, and a taskbar usually sits at the bottom or top of the screen eating up even more space (yes I know this can be changed but it’s not the default).
Documents are traditionally printed/read in portrait which is reflected on digital documents.
Programmers often rotate their screens to be portrait in order to see more of the code.
Most web pages rarely seem to make use of horizontal real estate, and scrolling is almost universally vertical. Even phones are utilised in portrait for the vast majority of time, and many web pages are designed for mobile first.
Beyond media consumption and production, it feels like the most commonly used workplace productivity apps are less useful in landscape mode. So why aren’t more office-based computer screens giant squares instead of horizontal rectangles?
I personally think portrait monitors, like a standard modern smartphone, would resolve most of these problems.
Also for programming, most IDEs make good use of the horizontal space and expect a roughly 16:9 screen where the IDE takes up most of the space on that screen. Not that you can’t just minimise the side panels but still, it’s a helpful feature of the software.
As for why portrait isn’t the default, I dunno, but if you start using a portrait monitor at work you’ll probably get some coworkers following suit if it’s such an improvement.