How does the rib cage and sternum protect the heart and lungs?

They prevent a slight bit of "cushion" to the heart, but no real structural protection. The ribs/rib cage provide the bulk of the protection to the heart (and the lungs, for that matter).

Because the lungs are soft, they cannot stop any significant penetration to the body that might hit the heart, whereas the ribs can provide this function, to an extent.

More times than not, the lungs can actually cause issues for the heart due to a build up of blood or air.