I use Safari, and quite often the right part of code blocks is clipped, and I have to scroll to see the text. Why doesn't the middle column scale when I increase the width of my browser window? There are big white areas to the sides, and in the middle there's clipped code. :(
The max width of Stack Exchange sites ensures the middle column's text remains nice and readable. There's lots of good answers on User Experience like this one which explain why longer lines aren't always readable. I'm on a 1080 pixel wide monitor as well, and I'm darn sure I'd rather not read SE with 200+ character width lines...