My eyes are aligned horizontally, not vertically.
Sure, I can make the case for more vertical space. But not at the expense of horizontal.
The only thing we use vertically is paper, and that's because we rarely consider the whole page in one go - only caring about one half at a time. And that makes it two pieces of landscape A5.
Books are portrait, I'll give you that. But you unfold them into a landscape A5-ish or large book with multiple columns (because of the difficulty of printing very near the gutter in the middle).
Children's picture books? Almost all landscape.
Movies? Landscape.
Photographs? Mostly landscape and certainly specified in landscape size and cameras are mostly designed for landscape operation (except when making portraits - for which we shockingly use them portrait!)
You have two eyes, one left, one right. Together they focus on the object of interest.
If you want a BIGGER landscape monitor so you can put a full A4 piece of paper on it - do that. Get it in landscape format and it will be wide enough to visualise two pieces at the same time at full height. That's not true if you flip the portrait/landscapes in those sentences.
Portrait displays have specific and specialised uses. And almost all of them leave horizontal space in everyone's visions (sometimes for a purpose, e.g. portraits without lots of side-art on them, sometimes because of cost - airport displays not being wider than necessary). If you fill that horizontal space, you get a landscape display of the same height that is suited for all purposes.
I can't see the case for portrait monitors for ordinary desktops at all except to "be different" or in very specialised applications where a landscape monitor of the same height will do twice as much.