I have lots of tabs open, and every other site I use chooses a legible width.
You think I'm going to resize my browser window narrower every time I switch to HN, and resize it wider every time I switch to a different tab?
Sites are designed. Legibility is part of design. Appropriate characters per line is part of legibility. Full stop.
Or you could just put it in a separate window of the desired width; maybe then you'd even have enough space on your huge monitor to see several sites all at once!
As I wrote the post above, I had about a dozen different windows open, all of varying sizes. Monitors have gotten much bigger, yet the users seem to have gotten worse at making good use of that space.
(Nor do I have a huge monitor, I'm on a laptop...)
Not disagreeing with your premise BTW.
...resize your browser window? The text is as wide as you want it to be.
On the other hand, I absolutely hate it when I want wider or narrower lines, and resizing the window either causes useless whitespace or a scrollbar to appear.