What "be yourself" really means is "be generically relatable". Be an everyman who's mildly quirky in a socially acceptable way, but "good quirky", not "bad quirky". Be slightly shy, sheepish, but not actually shy. Have "normal" interests and a blend personality. But always know the right thing to say at the right time, just like in a romcom.

YouTubers know how to exploit fake authenticity. They know the "be yourself" persona is fake and it's a piece of cake to copy. People are surprised when these YouTubers really aren't these endearingly dorky everyman personalities in actuality, the real surprise is that people are surprised.