It's a convenient lie that Normies spew in order to willingly remain ignorant to the utter state of the social and dating world. If they can force outward blame onto people "Oh you're just not being yourself" then they get to ignore the inherent and constant social issues that are slowly destroying society. It allows people to think that all their failures and issues simply stem from them having a fundamental lack of understanding about themselves rather than a series of outside instances that lead to his failure.
Oh you're hideously ugly? Don't worry there's a girl out there for you.
Oh you've got a mental issue? Tee Hee so do I but I've still have 50+ cocks, be yourself
Oh you've got crippling self esteem issues? So do I bro, I haven't gotten laid in like 3 weeks
Its the ultimate lie that Normies spew. "Be yourself" is nothing more than a platitude that they use instead of offering any real advice or genuine insight. What if 'Being myself' is sitting inside watching Hentai? What if it's being a massive cunt to people? What they mean is "Be what society deems to be an acceptable personality type and neatly fit into the part that we all act out on a daily basis, don't be yourself, be a constructed version of yourself". They just mean that you should go out drinking, enjoy the same shitty media, have the same shitty opinions and behave exactly like they do.
People throw it around to act like there's nothing stopping a person from banging a different 10/10 giga Stacy each night is simply their attitude rather than admit the huge factor genetics play in the role.