Light and sound travel at different speeds.
Light waves travel about 186,000 miles (297,600 kilometers) per second. The distance they must travel to reach our eyes from a cloud is covered in a tiny fraction of a second.
We say we see lightning instantly because our senses cannot distinguish the infinitesimal amount of time it actually takes for the light to reach our eyes.
The sound waves of thunder, on the other hand, travel about 1 mile (1.6 kilometers) every 5 seconds at ground level.
Therefore, we register the sight of lightning long before we register the sound of the exploding air caused by lightning.