He is an United States Senator, the most exclusive club on earth. He gets briefed plenty.
If you recall during the recount baloney, Klintoon wouldn't brief Dubya on National Security issues until the election results were official. I think he said in the effect that 'it wouldn't be fair to Gore'. And so Willard didn't brief either of their teams. But since Gore was in the administration anyway, it was really just a ploy to hurt Dubya's transition team.