Why does the ground jerk suddenly instead of sliding smoothly in an earthquake?

Because the ground isn't sliding smoothly in the first place: it's stuck. Along an active fault, the two sides are slowly dragged in different directions. But friction locks the fault itself. So instead of gliding past each other, the rocks on each side bend. This is the elastic-rebound theory. Rock near a locked fault bends like a twig, storing energy for years or decades.

Then one day, the strain is more than the rock can take. The fault breaks all at once. Both sides snap back almost to their old shape. Part of the stored energy turns into heat. Part goes into changing the rock itself. And part races outward as seismic waves, the shaking we feel. It's less like a slide and more like letting go of an overstretched rubber band.

The idea came from real detective work. After the great 1906 San Francisco earthquake, geophysicist Harry Fielding Reid dug into old land surveys along the San Andreas Fault. They covered the fifty years before the quake. He found that the ground had been slowly bending the whole time, by 3.2 metres in total. The earthquake, he concluded, was that stored bend snapping back. Much later, GPS measurements largely backed him up. Locked faults really do bend, often by a few centimetres a year, before letting go in one jolt.