Because not all content is suitable to be stretched out 16:9 (or whatever the screen ratio of your desktop monitor is). In fact, in my experience most web site content isn’t suitable for it. The default browser window size I use is closer to 4:3 for that reason. There’s also software to auto-resize the window to different widths and to auto-place it to different horizontal positions depending on the website, that you can configure for frequently used websites.
Only complain would be that its sometimes difficult to see the parent comment(s) when you scroll down.
HN could fix this quite easily by making the current parents sticky while you scroll.