The administration sold it on lies and misinformation, and a lot of people bought it.
Speaking as someone who was not only alive at the time but actively serving in the military at the time, you seem to forget Saddam himself was being conned by his own scientists who feared for their lives if they reported failure. Furthermore, the CIA believed they had WMD's, and the head of the CIA advised the Bush administration that WMD's were present.
So here you are, the President, sitting in the Oval Office. You've got a murderous thug of a dictator, someone who has shown no compunction about using WMD's against his own people when it suits him. His own services report having WMD's. Your own intelligence services confidently say he has WMD's. What do you do? Ignore all that?
Put this way, if you go to three different doctors and they all diagnose you with cancer, are you gonna say "nah, I feel fine, these guys don't know what they're talking about"? Or are you going to get treatment for cancer as if you actually HAVE cancer? And if afterwards when the chemo has made you sick as hell and all your hair falls out you discover you didn't really have cancer, are you going to blame yourself for making the decision to get treatment? Or are you going to blame those that wrongly advised you?
Based on your above comments, you'd have to blame yourself and hold those who wrongly diagnosed you as completely innocent. Not that that makes any fucking sense, but that's what you're doing.