The bits shattering are just aerodynamic panels, the main monocoque or chasis has remained intact, it does have crumple zones to absord some of the energy but is designed to stay in one piece to protect the driver.
It's not just the aero attachments shattering - it's the front rear and side crumple zones. Yes, the monocoque is designed to stay in one piece, however part of the design of making it stay in one piece is to sacrifice the rest of the car as an energy absorbing device. So.... you'll notice that the engine, fuel tank, wheels and all the other heavy bits on an F1 car (or IRL cart) depart the fix pretty rapidly in the event of a major incident, that reduces the cars mass and therefore it's inertia. After that the crumple zones offer progressive deformation to absorb the remainder of the energy until they shatter. At which point the car has hopefully bled enough energy for the monocoque to remain in one piece.
Which goes back to my original posting regarding energy management and that CF gives a huge amount of energy absorbtion compared with a similar mass of aluminium or steel.