Why do airplanes have never exceed speeds

The answer depends on how much you exceed V_ne. I do not have the information for this particular aircraft in front of me, so I can not say what will happen at a specific airspeed. Cessna spent a lot of money and time on engineering and flight testing to come up with the maximum airspeed (V_ne). There is a safety factor built in to this calculated and tested speed. One situation is flying level at a high altitude, then go in to a steep dive. This may cause an overspeed condition. One of the first problems as a result of overspeed, will start out as a vibration of control surfaces and controls. The chance for loss of control by the pilot due to mechanical failure or metal fatigue will increase exponentially, as does the airspeed. The C172 has airfoil designs meant for low speed. Depending on the exact plane and loading this could differ slightly, but the stall speed is 49 KIAS.
With its straight, high lift wing structure, the airfoil may actually completely loose its lift at a very high airspeed. As the airspeed increases, farther above the V_ne, the craft will basically start to tear apart and disintegrate.