Every website that’s mainly for displaying text (think into pages, blogs, Q&A) assumes your browser window is portrait like a phone screen. If I have widened my window I want the text to reach the edges, not float in a central column with masses of useless whitespace either side.
Well I disagree, because I find having to scroll up and down more often makes it less readable, and if I wanted it to be thin I’d make the browser window thin. Return to 90s websites where the site just gives you the info and how to display it is left entirely to the browser and the user.