They act like they don't even know they have a mess in their pants. They don't smell it. They don't feel it. They're oblivious to it. The answer is simple - denial.
Denial is one of the most potent of human ego defense mechanisms.
If he believes that he can't feel it and he can't smell it, then he doesn't have to deal with the problem. And he is not lying to you. He believes it. If you can't imagine anything much worse than dealing with the constant mess in your child's pants try to understand that to your child there is something infinitely more frightening and awful, namely, sitting on a toilet and having a bowel movement. The treatment is difficult.