Why choose wide monitors in a mobile first world?

I recently switched my main external display to vertical - mainly for programming - and started to leave it that way for email, browsing and blog reading. It got me wondering why monitors are almost always wide aspect and the "productivity" moniker is reserved for ultra wide aspect monitors.

I admit to not having spent significant time using ultra-wides, and am still writing this on my laptop screen, but everything from writing to scrolling restaurant websites is actually great on a vertical screen. UX today is always made to work well for mobile and having a giant mobile experience seems natural.