Why do economists believe that the average person is "rational"?
Rational choice theory, alongside methodological individualism, is the theoretical framework and normative foundation used by mainstream economists to mathematically model human behaviors in microeconomics.
This model assumes that the preferences of the average person are complete and transitive and not being shaped, determined, or manipulated by external factors including social institutions, socio-cultural environments and conditions, and power relations (such as educational system, mass media, family, etc.). Thus economists usually take preferences as given fixed and exogenous, and they assume that the average person in general has individual agency and autonomy to think for themselves.
This results in economics being seen as the closest of all social sciences to the hard sciences like physics.
However, a lot of scholars in the other social sciences especially sociology, critical studies, and criminology reject this, saying that the underlying premise of rational choice theory is unproven, or even pure bullsh*t. In their analysis of society, they claim that preferences are endogenous and the average people and public opinion are constantly manipulated by social structures to believe, act and behave a certain way often including "against their own interests".
Why do economists believe that people cannot be manipulated by external factors and other actors into changing their economic and business preferences? Do economists think advertisements cannot manipulate people into changing their own preferences?