In any field of study, people always start with the ideal scenario in order to eliminate external factors that could only add complexities. Then, when we fully understand the concepts, the calculations gradually evolve to more complex ones to cater to realistic problems.
The basic assumptions that economists do are the following
1. People always have preferences on a set of outcomes and this can be determined through the quantity and quality of their value
2. People always maximize utility or the satisfaction you get out of buying and using the product.
3. Businesses always maximize profit.