Why aren't computer monitors more commonly square, specifically in the workplace?
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?
the most commonly used workplace productivity apps are less useful in landscape mode.
They aren't less useful, they just don't take advantage of the extra space on their own.
A wide monitor allows you to put multiple windows side-by-side without the expense of an additional monitor though.
With that in mind; a wide monitor is useful for document editing, web browsing, media viewing/production, gaming, and can even be rotated (stand/mount permitting) for a tall view if desired.
Essentially if you want to use a monitor horizontally that's fine, if you want to rotate it vertically that's also fine, if you want to have equal horizontal and vertical real estate you're out of your mind.