Why? I also usually use it with three columns of text, and it works well at that aspect ratio.
What you really have to ask yourself is why 16:9 is a better choice than anything else. It wasn't picked because 3 16:27 columns is ideal for text. The main reason we have them is because it's easier for the screen manufacturers to have just one aspect ratio to deal with. With 16:9 we're forced to have three columns per screen, but what if I only need two? What if I want more lines of text per screen? Square seems like a much more obvious middle ground.
Not just that, but it was an average of aspect ratios considered for the aesthetic of video data. Not text presentation.
My browser tabs are on the left side of the screen and they are all written left to right. There is ample space on the sides of websites to fit this and show the entire tab title. Meanwhile, other people have tabs along the top of the screen that all show one or two letters followed by “...”.
My taskbar is on the right and consists entirely of icons. The only text on it is the date/time and that is also written normally and not rotated.
This is the setup that I was saying I don't understand the resistance against — not 90° rotated text. When people say “vertical tabs”, they are not talking about rotating the text.
The real problem is our screens are the wrong shape. 16:9 is a stupid aspect ratio for a computer monitor if you work mostly with text. Square is probably the best. Using floating windows (like classic Mac OSX) is an option, but for some reason people like full screen windows these days. A tiling window manager is another option to effectively divide the screen up into better shaped areas. I tend to have 6 columns across 2 screens which works well.