I don't know why we don't. Everything is connected--arts, sciences, math, history. The thing that connects them all is geography.
Here's a great place to start. It's put out by National Geographic of course. Every time we read a story with a specific place, anytime we discuss anything really, I'm always pointing to a map or a globe or Google Maps or something. Teachers teach geography as a seperate subject (if at all). Teacher ninjas sneak it into everything!