This would be in the many days before the invasion, there was a lot of moving around but we allowed them to move around.
Remember that Russian Convoy that was bombed on the way to Syria? Not saying they had WMD, but they were moving that way when the war kicked off, or right after. Russians were in Iraq helping them militarily. China helped them before the war get set up for it with air defenses, buried cable and tech.
During the bombing campaign, they were hiding, but there were plenty of areas that were not being hit. They were not bombing moving targets, but set ones, unless they got intel like when they tried to nail Saddam specifically.
The purpose for amassing WMD was military power. Saddam didn't think we were going to hit him. The UN had given 17 resolutions and he'd broken every one and kept on making threats. Saddam had powerful allies he thought would save his bacon, they couldn't. Those allies who actually voted that Saddam had WMD so they wouldn't look like they were dealing with Saddam never in a million years thought the crazy freak would push it to the end either.
Saddam thought he would be off the US stuff soon. With his 2 allies and France bought off to a point, 3 having veto power, it seemed pretty good from his point of view probably.
Why not use it? I explained possibilities in the other post, to tired to retype them, but using them would have sealed his fate. It's not like he was going to win that way. He was crazy, but he wasn't stupid, totally. He did misjudge though, I'm pretty sure he did that.
AND Saddam was attempting to buy yellow cake, that was a fact. The British verified that later, they had proof.