Why is Piggy certain that no one will ever find them?

Piggy and Ralph are the first boys we are introduced to in Lord of the Flies.  They quickly realize that no grown-ups have survived the plane crash that has left them stranded, and that apparently no people at all inhabit the island where they have landed.  The flight was filled with children; from what we gather in the first chapter, two adults were on the plane, and they are both gone.  One seems to...

Piggy and Ralph are the first boys we are introduced to in Lord of the Flies.  They quickly realize that no grown-ups have survived the plane crash that has left them stranded, and that apparently no people at all inhabit the island where they have landed.  The flight was filled with children; from what we gather in the first chapter, two adults were on the plane, and they are both gone.  One seems to have died:

“Where’s the man with the megaphone?”
The fair boy shook his head.

While Ralph is optimistic about the survival of the pilot, Piggy is more realistic:

“That pilot.”
The fair boy allowed his feet to come down and sat on the steamy earth.
“He must have flown off after he dropped us. He couldn’t land here. Not in a place with wheels.”
“We was attacked!”
“He’ll be back all right.”
The fat boy shook his head.

Piggy realizes from what he sees out the window when the plane is going down that the chances of the pilot's survival are non-existent.  He understands that they are now on their own.  Ralph, continuing to neglect the seriousness of their situation, then assures Piggy that his father, who is a commander in the Navy, will be right along to get them as soon as he realizes they are missing.  Piggy refuses to allow Ralph this delusion:

“How does he know we’re here?”
“They’d tell him at the airport.”
Piggy shook his head, put on his flashing glasses and looked down at Ralph.
“Not them. Didn’t you hear what the pilot said? About the atom bomb? They’re all dead.” 

Piggy is a child whose life has not been filled by fairy tales.  His dad is dead; his mom deserted him; his guardian-aunt placates him with candy; and his schoolmates bully and terrorize him.  He realizes the brevity of their situation long before the other children and is much more realistic about the possible consequences.