Theoretical propositions in introductory economics typically assume that people do not make wrong choices. Like most other assumptions, this enables to teach some core concepts and relevant math without worrying about 10 or 100 complicating factors.
Regarding your more general concerns, you can refer to debate about whether simple models are better to illustrate a theory or whether complex models should be preferred. (More likely, simple models are better for some situations and complex models are better for other situations.) Also, anything on the subject of "market imperfections" should provide a decent outline recognizing well-established reasons that main assumptions may not in fact be satisfied.
Much of the rest of the study of economics after core micro, macro and statistics courses deals with the extremely numerous exceptions and extensions upon basic theory.
To answer your question in particular, as opposed to what may underlie frustrations with the field in general, micro and macro must be considered separately:
Microeconomics is concerned with how individuals, households and businesses make decisions, in particular when faced with multiple options and differential costs and benefits.
Macroeconomics is concerned with understanding how this adds up at the aggregate level, mainly for the purpose of being able to provide a desirable level of macroeconomic stability which enables individuals and businesses to plan their affairs, while leaving plenty of space for old industries to die and for new technologies and innovations to be able to access financial, human and physical inputs to be able to grow.
From a public policy perspective, both micro and macro are to be used to improve the capacity and/or well-being of individuals, and the long-term production potential of the nation.
From a business perspective, these tools are to be used to understand competition and/or the macro context so that they can evaluate opportunities and risks in a competitive market. (It should be noted here that economics and finance are two distinct fields.)
From an individual perspective, knowledge of economics can contribute to well-informed, or alternatively overconfident, political participation. For example, many people who are not troubled by the simplifying assumptions that you recognize in introductory economics will then go on with extreme confidence in declaring that 'any tax cut is a good tax cut' (just check your textbook exercises on the effects of a tax on the sum of consumer surplus + producer surplus) or 'any government spending is a waste'. These declarations can be contrasted with (slightly) more nuanced argumentation such as 'the benefit of a tax must exceed its downsides' (and moreover the least undesirable tax is to be preferred) or 'government should only be involved in activities that are produced in insufficient quantity or quality by the marketplace' (e.g., building bridges and charging low or no tolls to use them).