Psychology is a science because it follows the empirical method. The scientific status of any endeavor is determined by its method of investigation, not what it studies. All sciences use the empirical method. Empiricism emphasizes objective and precise measurement.
Human behavior is very complex. To understand it, as best we can today, involves recognizing that humans are living and behaving organisms in a physical world. This tells us we must know science-validated principles from Physics, Biology, and Behavior Analysis, and we must use these principles appropriately to construct our understanding of behavior.
A foundation for using the principles of science is requiring that we deal only with Observable, Measurable, and Repeatable phenomena. This means that we must measurable observable behaviors and factors in the environment to Interpret, Predict, and Control human behavior, and we do not appeal to hypothetical constructs to explain behavior.
The science called the Analysis of Behavior has studied human behavior for almost a century and accumulated a powerful set of principles that can be used to “explain” most human behavior.