One reason that economists make assumptions when designing models is to:

One reason that economists make assumptions when designing models is to "exclude variables that do not add predictive power to the model".

Economists models are based on a series of assumptions, including that individual performers have consummate data about their decisions or that subjective human esteems can be estimated quantitatively. A few models even accept away competition, substitute merchandise and promoting. 
Economists create these assumptions, keeping in mind the end goal to apply testable speculations onto a generally deductive sociology. Without presumptions, quantitative financial models would not have the capacity to deliver any important results.