Piggy and Simon's possible knowledge of survival is underestimated by the other boys primarily because they are not taken seriously by the other boys. The majority of the boys view Piggy as an outsider; from the very beginning when Jack coined the name "Fatty" for Piggy, the larger boy was immediately discredited in front of the younger kids. As for Simon, even Piggy, a social pariah, views Simon as being "batty" for his love of walking through the jungle alone at night.
Due to Piggy and Simon's lackluster social positions within the group of boys, their potential knowledge of survival is discarded and ignored. Clearly Piggy is very knowledgeable, intelligent, and a better strategic thinker than Ralph, but seldom do the other boys seek his advice, because he annoys them. Simon's ability to move in the jungle and appreciate nature might also make him a good source of knowledge, but in the end, the other boys' close-minded views about both Simon and Piggy result in a total dismissal of their potential helpfulness.