Actually, Sarlaccs are in fact mobile. "Younger members of the species could move under the sands rapidly and catch their prey much faster, while older ones remained immobile, waiting for prey to stumble into their clutches."
Also, an interesting fact; only 3 people are known to have ever escaped the Sarlacc: Zorba Desilijic Tiure (Jabba's father), Count Dooku, and of course, Boba Fett! Actually, Fett escaped the Sarlacc on two different occasions. He must be really clumbsy.
