Why is this the answer?

The answer is library because this is a very fun word riddle that has a little secret hidden inside it. When you first hear the riddle, "What building has the most stories?", you might immediately picture a very tall skyscraper with lots of floors stacked high. Those floors are actually called "stories." For example, a tall apartment building might have twenty or thirty stories, like the first story, the second story, and so on. That is the first meaning of the word "stories" that the riddle wants you to think about. A library might not be the tallest building in your town, so this is the clever part of the trick! The word "stories" has another, more wonderful meaning that is key to solving this puzzle. A "story" can also be a tale or a narrative, like the ones you love to read in a book. Think about what a library is full of. It is packed with shelves and shelves of books on every topic imaginable. There are books about brave knights, funny animals, mysteries to solve, and amazing adventures that take you to faraway planets. There are also true stories about important people from history and exciting discoveries about our world. Every single book in a library contains at least one story, and sometimes even a collection of them. If you were to count all the narratives, tales, and pieces of information held within the covers of every book on those library shelves, the number would be absolutely enormous. It would be much bigger than the number of floors in even the tallest building you could imagine. Because of this, the library wins the contest for having the "most stories" of all the buildings. It is a marvelous place full of imaginative tales and facts that are just waiting for a reader to discover them. That is the genius of this riddle, using two meanings of the same word to point you to a building that is rich with countless adventures.