The reasons are mainly economic. First, because publishers are in business to make money from the books they publish, it makes little sense to give the product away. So when they do make their products available on the Internet, they often charge a fee. Many publishers of traditional library information sources are now offering web-based services. Libraries pay substantial amounts of money to give you access to these services.
Some publishers also offer subscriptions to individuals.
Fortunately, there are free resources out there, too. Many magazine and newspaper publishers, for instance, give away some part of their publications on their web sites - perhaps the current issue of a magazine or one week's worth of a newspaper. Increasingly publishers choose to give away their product and make money by selling advertisement space on their web sites. Futhermore, government publications are often free on the Internet since many government agencies disseminate information to the public. Another reason that you will not find everything on the Internet is related to the law of supply and demand.