In May 2002, the U.S. Supreme Court reversed the decision of the Third Circuit Court of Appeals that invalidated the Child Online Protection Act (COPA), which restricts children's access to obscene for minors material on the World Wide Web. In their decision (Ashcroft v. ACLU, No. 00-1293), five of the justices concluded that federal obscenity laws were not unconstitutional as applied to the Internet solely because obscenity laws require application of "community standards."
There's your problem. Laws have boundaries, the Internet doesn't. www.porn.uk (not a real site) exists in the UK, and is not bound by American law. www.porn.de exists in Germany, and is not bound by American law. www.porn.fr exists in France and is not bound by American law. I could go on, and on, but you probably still wouldn't get it.
The internet is a global medium with servers everywhere. Stamping out porn on the internet would be almost exactly like fighting fire ants. You can never really beat the ants. The best you can do is get them to move their hills periodically.