Why do techs like their computer monitor vertically rotated?

Two reasons. First I rarely write code that is long width wise. So being able to refrence things earlier in code without having to scroll is nice.

Second it actually allows and EPR/OPR to perfectly fit on the screen.

Yeah, I'm in the camp that does this too, but only a) if I have at least 2 screens, and b) if they are no more than a 16:9 ratio. And like already mentioned by others, it just makes coding easier because you can see more of it at once [1]. When I program on my laptop instead of my desktop, the former requirement is a problem since you can't flip that screen [2], and when I program at the office the latter requirement is the issue, as I have two curved ultra-wide 21:9 screens that just aren't conducive to flipping [3].

[1]: For the same reason, when coding on non-flipped screens, I usually have at least 2 vertically-split code windows/tabs open at once.
[2]: Before you ask, if I'm programming on my laptop, it's *because* I don't want to be constrained to the desk, so just hooking up another external monitor and flipping that is not the issue.
[3]: They also wouldn't fit right on the desk if I tried. But because of [1], I can often get 3 tabs on one screen, and 2 tabs plus the project hierarchy/watch/console on the other, so I guess it's fine.

I have 3 screens at my civilian work. 2 regular. On one I develop / regular use, on middle one I use for multi-tasking like keeping outlook always on screen. On third I use for reading documents vertically. Sometime I use it to test websites/apps that I develop for mobile browsers.