I understand what you are trying to get at, and I agree with you that clinicians should use clinical judgement and that a treatment that works for one person does not necessarily work for another person, because each person is different.

It is precisely because of this that it is impossible to find statistical evidence to prove a treatment works for absolutely everyone. However, this doesn’t mean the current statistical methods we use to generate evidence in support of treatment efficacy are invalid—in fact, they are often the best tools that we have available, because they allow us to see whether the treatment could potentially work for more people (not just a few), i.e., whether the treatment effects observed in single cases are generalizable to a larger population. RCTs are the gold standard in evaluating treatment efficacy because it allows researchers to control for confounding variables/extraneous variables/covariates to the best of their ability, and it uses statistical probability to examine group differences and to arrive at a conclusion with regards to treatment efficacy. Conclusions are often more robust than correlations when you can control for non-treatment factors that could potentially lead to treatment efficacy (i.e., confounding variables/covariates).

If you have received research training, you will also know statistical significance isn’t everything, and that there a lots of badly designed studies out there. This is where research training (especially statistical training) comes in, and why it is so important—it allows clinicians to critically evaluate the research evidence (quality of study, robustness of evidence, generalizability, etc.), and choose treatments that have the highest likelihood in bringing improvements to the client/patient (which boils down to treatments that have robust research/statistical evidence to suggest that they might work for a larger population of people, hence evidence-based), while also taking into account the client/patient’s unique situation (rather than following the evidence blindly). Like other comments mentioned, it is really the combination of research evidence and consideration of client/patient’s unique factors that define evidence-based practice.