Most airports are way more than 20 miles apart in the middle of the country unless you count those dirt road airports the crop duster guys use. This may be great on the east coast, where everyone is condensed together, but it won't fly here (pun intended).

They have flights here from Boise to the far east side of the state, but they are not direct so it's easier to rent a car and drive it. Also, when you get off the plane you still need a rental car anyways.