Notice how rarely anyone is argued out of a good excuse, and how often they are simply moved past it by a change in circumstance. None of this is new. Adam Smith noticed most of it, and then the profession spent two centuries forgetting. The usual story is that we are careless calculators. The better story is that we are careful about the wrong things. The point is not that people are foolish. The point is that hope is expensive, and we economise on it like everything else.
A better question
We describe the departure from the model as an error. We might equally describe the model as a hopeful guess about us. Consider a good excuse. It is a small thing, and small things are where habits live. A pattern this consistent is not a character flaw. It is a design feature, and design features can be worked with. Put a number on hope and it becomes a target; put a target on it and the number stops meaning what it meant.
