Google did test them, they thought they were making them permanent, and then the SEO community let them know why it was a bad idea. Essentially fixing something that wasn't broken, and in the course of it, made it WORSE for searchers instead of BETTER.
The idea was sound - CNN could add their favicon next to the page, and the user would inherently trust that the content was coming from that site.
However, get one spammer in there who puts up a fake favicon, and the whole TRUST relevance falls apart.
So at this time, favicons are used only in bookmarks and in browser tabs, no longer used on search results pages.