For me, for monitors and PC work, it comes down to OS UX. Sure, I can have all of my windows display at 4:3 with gaps either side, or on one side, but: (a) configuring that to be persistent is a pain, and (b) whatever fills that gap is either wasteful (empty space) or distracting.
The default on the other hand is to have anything maximised span the full width, which means that unless I'm reading a well-designed webpage (or reader mode), I'm inevitably straining to traverse a much bigger horizontal context with my eyes all day long.
Yes, 2560x1920 would be by far the preferred option of all 3, but if I need to choose between 2560x1440 and 1920x1440, the latter is a worthwhile compromise.
Dell 3007WFP and the Apple Cinema HD display were both ~16" tall. They're over 15 years old now. They were a expensive at the time, but they're still taller than most modern 16:9 and "27:9" displays available now.
>What I don't understand is why I'd ever want 1920x1440 (4:3) monitor over 2560x1440 (16:9) one, unless I'm really space constrained
I want a 4k 4:3 screen. If you're going to dream, why not dream big? 3840x2880 please. I have a CRT that syncs to 2560x1920 and I already enjoy it for a few odd things.
What I notice is that despite aspect ratios becoming wider, the monitors have pretty consistently also become taller in absolute measure. What I don't understand is why I'd ever want 1920x1440 (4:3) monitor over 2560x1440 (16:9) one, unless I'm really space constrained