Cleveland and Phoenix are about 1,746 miles apart by road. Split the difference and you arrive near Oklahoma City, the city closest to the halfway point between them. That puts roughly 19 hr 55 min of driving on the Cleveland side and 17 hr 37 min on the Phoenix side — the fairest single meeting point among the cities near the middle.
Over this distance most people will fly rather than drive the whole way. Oklahoma City still makes a fair, central place for Cleveland and Phoenix to converge, splitting the travel instead of asking one side to cross the country.
If Oklahoma City doesn't have what you're after, Kansas City and Omaha are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Cleveland and Phoenix.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.