This is a good point. There was certainly a lot of expectation before, during and after the war, that weapons would be found after the war quite regardless of whether they were there before it.
Some idle speculation as follows ...
The focus in the run up to war shifted substantially towards the ... err ... 'humanitarian' "Saddam bad => bombing Iraq good" argument, along with repeated attempts to make the Al-Qaeda connection stick. Almost as soon as hostilities ceased Wolfowitz was stating that WMD removal wasn't even a central war aim, and that the pre-war focus on them was just a bureaucratic compromise.
If you remember, way back before 1441 the reasons for going to war shifted a lot - from a starting point that was emphatically about regime change for regime change's sake, it then started shifting to arguments about 'humanitarian' action, links with Al-Qaeda and WMDs - the arguments seemed to shift arbitrarily on an almost daily basis. I think Wolfowitz may have been telling the absolute truth on this occasion - once they'd agreed to Blair's plan of trying the UN route they had to find a legal basis for war - and regime change isn't one. WMDs was the only option that could be dressed up as requiring a 'defensive' response to a clear and immediate threat.
From their point of view, I think the post-war strategy of trying to distance themselves from the WMD thang after the fact is back-firing badly. I'm not sure they have any real concept of the extent of anger out there, and also the extremely reluctant and highly conditional nature of the 'support' (or rather lack of overt opposition) from people who decided they simply had to take their word for it on WMDs and support the troops when war was declared. I think also that when they first implemented the strategy of retrospectively changing the war aims they didn't know quite how badly it was going to go - it would be much less of an issue, and thus a much sounder strategy, if the massively over-optimistic predictions of smiling Iraqis welcoming them with open arms had come true.
It was probably a bad miscalculation - but it must have seemed much the safest option at the time. How easy would it really be to smuggle credible quantities of WMDs in when the place is crawling with troops, not all of them terribly impressed at being there and many, perhaps the majority, with a healthy scepticism about the reasons for the war (the military on both sides of the pond were vocally hostile until the invasion was utterly inevitable).
I guess this latter point is similar to the argument put forward regarding 9/11 - they simply could not afford to get caught in the act. There is a critical difference though:
Planting WMDs in Iraq would be a government-level, covert-but-official decision - the risk assessment would be taken very seriously indeed. If (and I mean if) there was some complicity in 9/11, I seriously doubt that even Bush would have known about it - he's the monkey, not the organ grinder - and it's way too out there to be anything other than a group of "crazies" with ambitions and the right connections, champing at the bit after 10+ years of planning and 8 years of Clinton - finally in with a chance after successfuly stealing one election, acutely aware of the timing of the next one and the 'need' to have Iraq safely over and done with in time to capitalise on it.
In addition, of course, they may well have achieved 9/11 by manipulating Al-Qaeda or a similar group, in which case the risk of getting caught would be much less; the people putting the plan into operation wouldn't have to know who the 'client' was. And the "crazies" (as they were openly referred to in Bush Snr's time) would have a very different approach to risk assessment than the government. It's a pretty all or nothing strategy for them - the PNAC plan is hugely risky, and they know it even if they don't fully acknowledge the extent of the risks - but the alternatives are either imminent economic collapse or a rational economic policy - both utterly unacceptable, especially when it can all be avoided by going to war.
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