Why does theory not match reality? in economics

Economic theory tries to represent reality using models.

Models are simplifications of the real world. A set of assumptions is used to build that simplification. Models are constituted by a group of variables, and mathematical relationships are established among them.

Even tough the simplifications contained in models entail certain limitations, economic models are useful to draw conclusions, starting from a simpler scenario, which afterwards can be applied to analyze complex real structures theoretically.