Maybe tall people are precisely as confident as they should be, and it's short people who have issues. e.g. "Napoleon Complex". I've personally met more than a few people who fall into this phenomena. It's not a trait that's compatible with good leadership skills. As for why this might be the case at all, I'd guess it has something to do with us being glorified apes.
> In 2018, evolutionary psychologist Mark van Vugt and his team at the Vrije Universiteit Amsterdam found evidence for the Napoleon complex in human males. Men of short stature behaved more (indirectly) aggressive in interactions with taller men. Their evolutionary psychology hypothesis argues that in competitive situations when males, human or nonhuman, receive cues that they are physically outcompeted, the Napoleon complex psychology kicks in: physically weaker males should adopt alternative behavioral strategies to level the playing field, including showing indirect aggression and coalition building.[10]
Of course these are generalities and you can find many exceptions, but the same is true of "managers tend to be taller"; that too is a generality with many exceptions.
> In 2018, evolutionary psychologist Mark van Vugt and his team at the Vrije Universiteit Amsterdam found evidence for the Napoleon complex in human males. Men of short stature behaved more (indirectly) aggressive in interactions with taller men. Their evolutionary psychology hypothesis argues that in competitive situations when males, human or nonhuman, receive cues that they are physically outcompeted, the Napoleon complex psychology kicks in: physically weaker males should adopt alternative behavioral strategies to level the playing field, including showing indirect aggression and coalition building.[10]
Of course these are generalities and you can find many exceptions, but the same is true of "managers tend to be taller"; that too is a generality with many exceptions.