-not everybody has the patience to read bulky books.
-the topic of the book might not of interest to the reader thus he will not be able to concentrate.
-from every new word whose meaning you are not aware of, you will have to refer to a dictionary.
-with books you can learn new words and expressions but are not able to learn their pronunciation.
-mere reading of a book will not help, the words and phrases will vanish from your mind if you use them in your speech and practice them.
with watching Movies
-In movies the conversation could be very informal, so a viewer may learn informal expressions and slangs that good formal speech.
-The conversation is at a faster pace, so it becomes tough for a non native speaker to understand the accent. He will have to pause and replay a scene many times to understand what exact was said.
-mere watching a movie will not help, the words and phrases will vanish from your mind if you use them in your speech and practice them.