Most monitors are 16:9 with the 16 being the horizontal dimension. It's also difficult to read very long lines of text, so my browser windows tend to be narrow and tall. When I use the developer console, I pop it out into another window and put it next to the original window.
> Most monitors are 16:9 because horizontal space is more important than vertical.
Most monitors are 16:9 because that aspect ratio fits certain popular entertainment media content that people consume, and a significant usage of general purpose computers is consuming that content (and, also, a significant use outside of general purpose computers of displays is for consuming that content, which effects the economics of producing panels.)
That doesn't necessarily reflect the relative value of horizontal vs. vertical space in other applications.
With a 1080p display, my windows only use half the screen, and I hardly ever feel that it's too narrow. What pages are you browsing that benefit from being wide?
