I think a lot of people want to keep up with the latest media, and there are lots of social reasons to do so. The latest TV shows, movies, and books will be the ones your friends are watching/reading. As xkcd eloquently put it: http://xkcd.com/606/
This points out one more factor: content prices decrease dramatically after several years. Videogames go from $60 to $10 on Steam; Movies end up in discount bins; books end up cheap on amazon or from used bookstores.
Books are an interesting example: when they are first published it is difficult to acquire them at a local library (because lots of other people want to read the same new book), but after some time (usually much less than a year), they are easily acquirable for free. If the library model works, such that authors are satisfied with the profitability of writing books, then reducing copyright term to a few years is probably reasonable.
EDIT: I personally think somewhere around 10 years is probably more reasonable, but I think the discussion around a more extreme 2-5 years is interesting, and the current term length is ridiculous.