One of the most painful parts of Economics is how many mathematicians there are in the field. They treat discovery science as hypothesis science, and try to force reality fit their model - instead of gathering data about the real world first. Thus, we get models of Marginal Demand that don't include "not having enough money to buy something".
Edit: if this comment is unhelpful, please suggest an update.
Edit: if this comment is unhelpful, please suggest an update.