Do people really know books?

When onesome asks for good book suggestions, why do people also mention the books from the series twilight and harry potter? Is it because those are the only books they have actually read? Is it because they have read no clics from famous authors like Fitzgerald, Hemingway, Joyce, Nabokov? If people were to give suggestions about good books, shouldn't they at least have read a good range of books that's more than just those from the two young adult series? Seriously, why do people claim that these books are the best books ever when they have read so little?