I don't get it.

We have only 1080px in vertical, part of which is also used for Taskbars, titlebars and toolbars in most cases. Then there is this trend of sites not using most of the horizontal space for main body text.
So, what reason do we have to not use the wasted side-space and instead congest the already low vertical space?

I would understand if it were a mobile-only site or if you were explicitly talking about the vertical version of it, but even for 4:3, I won't consider a sidebar to be a bad idea, unless perhaps, it was German.