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?
It’s easier on your neck to look side to side than it is up and down. So to get more screen real estate it makes more sense to go horizontal. Anecdotally, I constantly have two documents or a document and a web page open next to each other on one monitor. The landscape framing works really well for that.