I've been working a lot with Bootstrap lately, and I've realized the glory of a site that can scale up and down easily. Is there any reason StackOverflow would choose not to do this? It has zero effect on the site when the window is maximized, but when the window shrinks, it keeps the site from growing a pesky horizontal scroll bar. I use StackOverflow primarily in half of my screen (by autosizing the window with Windows 7), and it could be beneficial to have all the content fit within the boundaries of the screen instead of escaping the dimensions.
If you look at the width of the content it matches perfectly a 1024 pixel wide screen. Most websites do this now (I run dual monitors at 2048 width for this very reason). I don't think many developers even consider it now during the design process because there is little demand and little reward.