That's not fairly self-evident at all, that's a generalization. I try not make them, though nobody's perfect. I've just seen lots of real examples over the years that bust the stereotypes, as well as being an example of it myself. I've gamed with folks who were super level-headed and competent despite not even being half-way through high school. One of my zombiegame striketeam leaders was 15 when she started commanding, and she ran one of the most successful and largest teams we had till she graduated high school and went to college. Also gamed with adults who were the most petty and wound-up people I've ever met and couldn't win or lose or even play the game with any grace whatsoever. I left an RPG group because the two adults were arguing for real about who was going to fly the pretend spaceship even before the game itself started.
As far as yourself not being able to play villains, then acting is not your career path. If you ask actors though, many of them LOVE to play villains, because they're frequently more interesting to portray than the hero. They're usually nice people IRL, they just pretend to be terrible for fun. There are also actors who play mostly heroes, who are purported to be such complete holes IRL that their own heroic characters would despise them. Acting isn't representative of anything but how well you can get yourself into someone else's shoes.
None of this denies the existence of salt miners, but it also doesn't deny the existence of surface-level salt mines. They feed into each other. Meanwhile they both cloud actual discussions with their crowing and bleating when both of them just need to be mainly ignored.