They used to; there's a legend from the Tusken natives that the dragons are all descendant from one, named Krayt.

[ He paused a moment, glancing over to see that he still had everyone's attention. With a sheepish shrug, he continued on: ]

Krayt was a ravenous creature that ate the livestock of a village every night. Desperate and worried that they would soon run out of bantha's to satisfy the beast, so they sent their most skilled warrior to slay the dragon. But she never returned, and when the dragon returned, he had acquired a new taste for the flesh of the villagers.