This is effectively the approach of Athens. Although familiar and portable, usernames and passwords have lots of problems. They are not very secure. Users can trade them. For example if publishers gave out usernames and passwords that could be used from anywhere there would be nothing to stop authorized users giving or selling to non authorized users.
Users tend to have a multiplicity of passwords, which they find difficult to keep track of.
Administering a system of individual usernames and passwords for thousands of users is complicated and time consuming. Only machines registered by the University with the publisher are allowed access to resources. The user does not have to try and remember a username and password. If a user uses an ISP other than the University because ISPs generate IP addresses dynamically the user has no IP address that could be registered, even if publishers and libraries accepted the administrative burden of registering home users IP addresses. IP based authentication protects the privacy of the individual on the whole, because generally users are logging on to different machines - but this also means that it is impossible to find out how different groups of users are using resources.