Most websites do that so they can avoid horizontal scrolling (a web faux pas) on smaller screens. A popular design trend (at least, last I saw) was 960px wide designs, so as to fit inside a 1024px wide screen nicely. A better solution to a fixed width design that's narrow on larger screens would be to use reactive/responsive design to make the most of the screen space available.
For responsive design tutorials, check out http://www.smashingmagazine.com/responsive-web-design-guidelines-tutorials. It's very simple when you get down to it, you use @media
queries to apply different rules to elements depending on how big the viewport is.