The equivalent of the trusty card catalogue or its electronic equivalent in the traditional library has not been developed for the Web. A grocery store analogy would be if all of Safeway were to become one huge discount table. Some people would know where some of the things are but nobody knows where everything is. And tomorrow, some of those things would be missing or moved to a different spot. Second, Web materials can disappear in an instant.
Files are erased, computers crash, power fails and links are broken because files are moved.
Unlike the library whose mission is to serve as a reliable, long term archival source of printed information, most providers of Web materials have no such mission. Most academic institutions have no long term commitments to archive materials posted on their websites. In spite of this lack of commitment is the spreading perception that the Web surpasses the traditional library as an information source and thus make it obsolete. This perception will only increase the financial pressures against libraries, further diminishing their collections. Note that schools, K-12 and college, are increasing Web allocations while their libraries suffer relative decreases.