Behavioral science is too hard to get right. The effect sizes mostly don't seem to be big and it's too hard to run a trial, so to keep a career going scientists are forced to aim for some sexy outcome which isn't there from an underpowered experiment. But almost nothing really matters in that field. It's as good as plastic recycling.

Still, we are forced to study it because there are interventions that have worked in the past: cigarette smoking, teen pregnancies, etc.

And it's useful to know what made those so successful. But maybe the answer is luck.