In Lord of the Flies, why don't Jack and his hunters take the conch when they attack Ralph, Piggy, and Samneric?

After Simon is killed (Chapter 9), even Ralph starts to doubt the power and effectiveness of the conch. Simon's death is significant in that it shows Jack's tribe moving further into savage behavior. In Chapter 10, sitting with Piggy, Ralph stares at the conch and laughs when Piggy suggests they use it to summon a meeting. Ralph realizes that the symbol of reason (conch) now has no power over Jack and his followers. Jack rules by force and intimidation: not by using a tool of organization. 

By this point, most of the boys have defected or been scared into joining Jack's camp. They really have no use for the conch any more. Considering that the conch symbolizes order and civil behavior, it is fitting that Jack doesn't see a need for it. However, they could still use Piggy's glasses to start fires. Jack's primary concern, other than ruling like a tyrant, is hunting and meat. Therefore, the subsequent need is something to start fires to cook the meat. 

When they are attacked, Piggy thinks they have come for the conch. Piggy still thinks (or wants to believe) that the conch has some unifying and/or civilizing power. But Jack has made the transition to a savage ruler. Now, all he wants is the glass.