The point is that most websites have sidebars, but this one doesn't.
Let's say my preferred text width is 10 units. Due to sidebars, most websites require a browser window width of 20 units to show me 10 units of main text.
Then I visit this person's website and get text 20 units wide and it's very uncomfortable to read.
The text width I like doesn't change but the browser window width I need does change, and I don't want to change it when navigating around, to be honest.
I guess I could use the browser functionality to inject my own CSS rules for specific websites, then I could change the single-column no-max-width websites to show me a sensible text width.
But I also suppose that I'm not alone with this preference. I believe there have been studies about a suitable line length that is easy to read, and this is reflected in printed media. So those websites ask everyone to make such changes. That doesn't seem right.