Because it doesn’t point to a “good” outcome. It just boosts self-esteem.

Sure you might encourage someone to do something they wouldn’t. But you have no modality to indicate that the new action would be “good” regardless of how you define “good”.

Culture-based behavioral regulation is a reliable way to ensure homogeneity of a unified set of cultural values, which we could reasonably call “the collective good”.

Would you tell Hitler/Stalin/Mao to “just be yourself?” Of course not! You’d be encouraging evil. They didn’t need encouragement because they were malevolent narcissists. But that begs the question, how did they become narcissists? I’d posit it’s, at least in part because someone, at some point encouraged them to “just be themselves”. And look how that bloody went!