Producing a web design that adapts to screen size isn’t just about saying “ooh – you’re on a desktop, I’m going to give you a full-width page.”
On the desktop, windows can be resized. They can be full-screen, half-screen or any other shape. I can easily hit Windows key and the right-arrow and *bam* the browser is now taking up half the screen. Great for multi-tasking, but a pain in the backside if the web designer decided their creation had to be seen full-screen.
Assuming the user will always oblige by running your website full-screen is dangerous. Thankfully truly responsive websites are taking over.