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.