You either have your monitor set to fairly low resolution (1024 pixels wide or less), or you just happen to keep your browser window sized to about 900 pixels wide.
Resize your browser to be bigger and you will see that the content no longer scales to use the entire window.
And if you resize your browser smaller, at some point a scroll bar will show up along the bottom. This never used to happen - before the content would redraw itself to use the full width of the window, however wide it is.