My guess is EMS is probably not primarily auto accident response so population density/tax revenue has more to do with location of fire stations. Along rural 5/99 in California often see county fire responders, versus seeing municipal stations respond to situations closer to towns. Somewhere along the way EMS has switched from independent service seen in that 70's TV show about EMS in my childhood to almost always being attached to a fire station. Anecdotally from a suburb in Silicon Valley I hear the local firestation truck sirens go off multiple times daily when working at home, and there are an average of zero fires a day anywhere near my home.