That's because Saddam was given the perfect opportunity to get those things outta Dodge while we were screwing around with the UN in the months leading up to the war.
Why in the heck isn't the public reminded about the satellite video that Colin Powell showed the UN and the world with those convoys of trucks leaving known WMD sites in Iraq and headed to Syria?
Yes. And why Bush doesn't actually come out and say those things boggles my mind. All he has to say is
Saddam had ample opportunity to hide those weapons, we have found planes buried in the sand and we may never know what was there or not there. But we do know that he had WMDs because he used them on his own people and we found them, - we just have not found large caches. etc
I imagine we will eventually find that Bush regreats taking Powell's advise to go to the UN with a thin case about WMN, rather than taking the bull by the horns and saying our purpose was to remove Sadaam.