What do I look like, Mr. Answer Man?

The Washington Post reports that White House press secretary Tony Snow frequently responds to questions with "I don't know."
"When Snow took over as White House press secretary earlier this year, reporters found it refreshing that he was willing to admit when he didn't know something," wrote political columnist Dana Milbank. "This has become rather less refreshing as Snow, while claiming access to Bush's sanctum sanctorum, continues to use the phrase -- more than 400 times so far in televised briefings and interviews. Sometimes, it seems more of a tic than a response; usually, it's a brushoff."
OK, the guy's job is to answer questions and he doesn't know anything?
Why would you hire a press secretary who doesn't know anything?
I don't know.