I'm not trying to blame users, I'm saying, regardless of what started the conversation (I could care less about this design), that people should perhaps stop maximizing everything on large displays, if that's something they do, as it may be counterproductive.
As for websites providing a singular column view, and sized specifically, there are a lot of reasons for that, and there are also reasons why a multi-column view makes little sense (which of the possible ways does scrolling work if the content can't fit on the screen?). Multiple text columns worked for print media because they had a fixed format, and were also size constrained in pages, meaning they were heavily incentivized to put as much on a page as they could. Text books often have some similar constraints (because there's so much information contained), so rather than 700 smaller pages, they use less pages but multiple columns, but even then they often try to break it up with images and diagrams so it's less common. Recreational reading is probably the closest to the optimal form of comfortably reading, and that uses smaller format singular column pages.
That's not going to translate exactly to the web, but something that is taken into account is that longer line sizes make for things harder to read. With a maximum number of characters in a line size, to fill a window your choices are to either leave white space or use larger characters unless we want multiple columns, but those are harder to read and follow as well. Larger characters will result in less information on the screen at a time, and above a certain size is probably only minimally better than smaller characters, so what we commonly have is white space, for numerous reasons.
There are many good reasons to have a singular column of specific size. People complaining about all the unused space when they can easily rectify that by zooming or resizing is silly, given all the other constraints that make that format optimal.