By default, all browsers initially open in 100% of screen width. To resize the browser requires a manual change by the person viewing that browser. Unless there is a compelling reason to change the default opening width (such as a stretched out site) practically everyone keeps the default view (100% of screen width).

Again, it is a user-efficiency consideration. By making your site 980 pixels wide you remove the need to alter browser size in over 95% of views. Every EXTRA action needed by your users contributes to inefficiency.

Consider too that people now browse in TABS... flicking from one site to others in quick succession. IF they have to re-size their browser just because YOUR site is affecting the look of the other tabs, they will stop looking at your site, and look only at those that offer them less hassle.

As I said... this is detailed research done by Forrester and eConsultancy. You are perfectly free to ignore it, of course.