Note that in RL humans have it quite easy and safe. People have food, shelters, help, they don't kill each other that often and they're safe from predators.
In a D&D world a human or elf has to somehow survive. Villages are attacked by monsters or bandits on a regular basis. Traveling is dangerous, but often needed to get by (by selling your merchandise in a city or trade between cities). People aren't even safe in the towns as there are assassins, thugs, gangs, evil organizations and cults, monsters that disguise themselves, etc.
I really am not surprised that a D&D world can have an unusually high % of higher level population. It's dangerous.