The majority of text displayed on a monitor only uses a small number of pixels, which emphasizes the edges where it doesn't cleanly intersect all of the component colors of the white background. #000 on #FFF emphasizes the colorful artifacts. We don't control what monitors our users own. Taking a step away from the most extreme possible contrast makes our UI's look a lot less like rainbow bleeding failure on the uncalibrated 1080p 29" monitors commonly used by executives making software expense decisions.