It's more that a lot of people are more inclined to trust a leaked NSA document that claims NSA does the type of things that people already believed NSA does, including when it implicates Google, than a Google PR denial of a very specific interpretation of the same.

In the absence of more evidence about what exactly PRISM does, what we have is guesswork, but guesswork where believing some interpretation of the NSA documents becomes easier the more revelations of extensive NSA surveillance via other channels that are coming out.