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’m not excusing poor design, I’m saying in many cases there is no UI design you could implement to use the full space. You have to accept that somethings are a different form factor and either use the extra space for something else yourself or accept that it’s just unnecessary space in this particular use case.
I am saying “just use the extra space for something else”, because that’s exactly what it’s for. You have a wide display so that you can display wide content or several pieces of tall/square content. Expecting EVERYTHING to conform to a single form factor is insane.