Google will only find you the books you are asking it to find. Consider a high school student looking for information on a certain event in World War II. Sure, she could google "world war 2", but then what? If she doesn't know what she's looking for, she's completely lost from that point on.
A good librarian, on the other hand, could help our hypothetical student and guide her between the different sources of information: encyclopedias, litterature, magazines, maybe even movies. Sure, nobody's perfect, but it's much better to have someone to talk to than to be presented with a prompt for keywords, especially when you don't know what those keywords might be. The job description of a librarian, simply put, is wider than "take keywords, fetch books relevant to those keywords".
Google might be good for when you know what you're looking for. Librarians is for everything else.