It is always hard to say why a large company does or does not do something, but do note the next line of the law which mentions:
other contact information which the Register of Copyrights may deem appropriate.
In general the purpose of the contact info is to allow claimants to file claims of infringement with the agent. If that purpose is served, I doubt anyone will sue over the exact form of the contact info provided, but I cannot be sure.
The wikipedia article that i linked to in my previous answer also says:
There is a common practice of providing a link to legal notices at the bottom of the main web page of a site. It may be prudent, though it is not required by the provisions of section 512 of the copyright law, to include the designated agent information on the page the legal link goes to, in addition to any other places where it is available. As long as the site gives reasonable notice that there is a method of compliance, that should be sufficient. Once again the courts have not ruled on the technicalities of posting of these notices.
It seems that the law in these matters is not fully tested in the courts.