This is a great observation and there's an obvious answer you may have not made the connection on: it's cheaper, more efficient, and saves lives.
Many midwestern states, because of their large land area, face a bit different problem than compact cities: you can only get some place so fast by highway. Furthermore, because of the large distances, it could be too late when certain first responders are dispatched and the information that was given over the phone was incorrect. (Think of software engineering, how many times have you been given the correct and complete requirements the first time?)
Instead, most fire departments not only have the training to hand the exact same emergencies as the EMS, but they also carry the same equipment onboard the fire trucks.
Another scenario, lets say fire was dispatched to car accident because smoke was reported 20m from the city, and when they got there, someone was bleeding severely. If it were me, the answer I wouldn't want to hear is "oh sorry, can't bandage that, we'll call for EMS". Worse, imagine the fire department was on a call like that and a high rise caught fire, 5m down the road. If they didn't take the fire truck, they'd have to drive all the way back to the city, grab the truck, then drive all the way back. That's why you see that all the time, it's the safest thing to do.
And fire departments go way beyond putting out fires. There are departments in cities that are trained in swift water rescue, electrical hazards, gas/chemical leaks, and many more. Think of fire as general rescue workers, rather than putting out flames.
Source: Firsthand volunteer firefighter work