To some extent I agree with you that behavioral science/psychology needs a unifying theory. Attempts at this have been made and are still alive and well. For example, take a look at Radical Behaviorism [1]--the philosophy of science behind (applied) behavior analysis.
The overarching goal of behavior analysis is to treat the study of behavior as a natural science. That is, to identify orderly relations (i.e., laws) between behavior and the environment.
The overarching goal of behavior analysis is to treat the study of behavior as a natural science. That is, to identify orderly relations (i.e., laws) between behavior and the environment.
[1]: https://en.wikipedia.org/wiki/Radical_behaviorism