Maybe not every human was a hero
We know that undeads are dead that came to life thanks to necromancy. But every single hero that we know exist has the exact same type of body. So why there are undeads bigger than humans or villagers? There are husks (in Dungeons) wither skeletons and necromancers. Maybe it's just Mojang that is too lazy to add different type of heroes, because they should do again every animation, but if they aren't maybe most humans were basically villager without the nose, and not every one was a legendary hero able to take down armies, monsters and entities thousands of years old, maybe most of them were similar to the heroes in Vanilla, and only some of them became then or was already a hero. Maybe that type was stronger, or somehow they managed to survive from the nameless kingdom rise / fall and they became stronger then, maybe with the tower, or maybe again they were part of the nameless kingdom, so they don't get attacked