I have an animated SVG on my README that is rebuilt once a day to include the weather and day of the week. Built during jury duty a few years ago :P https://github.com/jasonlong
I can't speak for everyone else, but in this case we were hoping to create something that could stand on its own and (hopefully) grow over time as more datasets are added. The domain wasn't registered just to get on HN - the idea lent itself to a simple site of its own, so why not use a fitting domain name?