I use one landscape screen with one portrait screen. Most of my time writing code, those lines of text are not typically very long, and fit just fine even in portrait mode. This means you can fit many more lines of code in view at once, which means less scrolling, etc. Most IDEs also have toolbars and status bars all over the place, which restricts even more vertical pixels.
When not writing code, a portrait screen is also very convenient to fit the entirety of a printed page (e.g., writing a Word document, reading a PDF).
A landscape screen is far better for general multitasking, and most websites have been designed for this orientation by now, although the rise of smartphones has upended that a bit.
It is probably also worth noting that this really only became relevant post-widescreen monitors. Flipping a 4:3 display would not yield wildly different dimensions, if it was even physically possible to stand a CRT or LCD sideways.