People rarely make giant mistakes for only one reason. You pointed out Saddam’s mistake. The WMD’s he did have were either buried or sent to Syria. The whole world knows it. but the left ignores it and Bush allowed that. That was one of the causes of his failure.
I'm hoping GWB addresses it in his book. I've always believed that the Russians were up to their necks in getting the WMD out of Iraq to hide their complicity with Saddam. That might have affected what info Bush could have made available.
Bush had the annoying habit of doing what was the best for the nation rather than what was best for himself politically.