For example, even though it has been published in 1953, Watson and Crick's DNA paper is still being cited. That, of course, does not mean it is actually being read. The DNA structure is now a part of every Biology 101 curriculum, but I'll be very surprised if more than a handful of the people studying it in the last decades have read the original paper.
Yet, the paper is still being cited. Same with other famous papers, books, etc. The classical paper citation has become an entity of its own, having been detached from The Paper. The Paper has been reduced from a meme complex to a single meme which carries on and on.
So, citation of a classic paper, especially an on old one, is usually more of "I know what those guys did and I appreciate it" than "I've read the paper and picked up something useful."