You go to the Web on a certain day and the item is there. The final problem is quality standards. Authors approach a publisher who has a legal obligation and a professional reputation to produce a quality product. Librarians rely on consistent quality from reputable publishers to set the standards. One thing books had that resources on the Internet do not have is the accountability of a publisher.
Publishers have a legal obligation to print the verifiable truth.
They edit the content, structure, and grammar of their publications. They also verify the sources mentioned. So, this brings up the issue as to whether the Internet is even worth the time to catalog due to the varied quality. There are many people trying to develop projects with the goal of establishing standards for all to use. The fact that there are so many efforts is a real problem in solidifying consistency.