To paraphrase Douglas Adams: Research libraries are big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big they are. I mean, you may think the book store down the road has a lot of books, but that's just peanuts to libraries.
Think of it this way: every time you use an academic research library, you are delving into the collected knowledge of the entire human race. That's 5000 years of written material, by millions of authors using thousands of languages. Books and journals just scratch the surface. There are clay tablets, scrolls, manuscripts, maps, newspapers, microfilm, videos, sound recordings, and a plethora of digital formats. UCLA alone has over nine million volumes, but if you count all the things we can provide access to online or via interlibrary loan that jumps by orders of magnitude.
Not only is there a lot of stuff, it's growing by leaps and bounds every day. Academic publishing is on the rise, and new journals start up every year. There are whole new categories of information like hypertext and GIS. It's a jungle out there, often with few roads or signposts to guide your way.