The point is that we don't have evidence right now. Lack of evidence is not evidence of lack.
We know that the stockpiles did exist. We don't know what became of them. If they were destroyed, fine, but it's not enough to say that we didn't find them. We need to be able to say (and prove) "this amount of anthrax was destroyed by this method at this location using this equipment on this date; this amount of mustard gas was destroyed by this..." and so on, until the entire stockpile is accounted for. Short of that, we need to act as if those weapons still exist. That was true a year ago and it's true now.
I suppose it's possible that Saddam had all that stuff destroyed when we weren't looking, and failed to keep any record of it, thereby causing a war. He should have been more careful in his bookkeeping. But that's his bad, not ours.