A lot of people use browser tabs, which allow you to use the same window to view different websites. And a lot of websites use highly different font sizes, and layouts without sidebars or with sidebars of varying widths. Combine those facts, and you'll see that if people tried to size their browser window to the desired width, they would have to resize it every time they switched tabs to a different website. Not ideal: it's better if sites include styles to default to a readable column width given their own specific layout.