I'm going to put it down to coincidence, based on an admittedly very superficial look through eggs.js, which supplies the zoom effect.
The map image, https://cdn.sstatic.net/Sites/travel/img/bg-map.png , is inserted as a background and positioned an arbitrary 750 pixels to the right so that it is not blocked by the input textarea. This positioning is not affected by the width of the window or screen at loading time.
The map is not intended to be completely accurate, of course, and is cropped in a way that excludes the polar regions. As such, the north-south midpoint on the map is a good 32° or so north of the equator. Additionally, part of the background is obscured by the border bar that runs across the top of the footer.
The image is both resized and repositioned based on the scroll position, and since the longitudinal center of the map is fixed, and the latitudinal center is biased north, you could get the impression that Chicago is roughly the center, but it's hard to say whether that was intentional or not.
From a U.S.-centric perspective, Chicago isn't a bad choice for a travel site, as it was the country's most important transportation hub for at least a century and a half, starting with steamships, then railroads, then passenger airlines.