The idea that you can get more powerful just by killing a bunch of things makes sense from a gaming perspective for the PCs, but not necessarily from the world perspective. Why should a cleric get better spells because they beat up some orcs? How does a mage learn the mysteries of the universe in a field doing the same fireball over and over? Or at the very least, why does it make a difference whether the fireball is cast at orcs or an empty field?
That question was really answered by Gary Gygax in 1979, other people have been coming up with their own explanations that have different advantages and drawbacks depending on if you want to have an 11 year old High Priestess or not.
From a different perspective, demanding the old priest that can raise from the dead or the ancient archmage have adventured, PC-style, seems limiting on NPC concept.
Special snowflake characters should be special exceptions not the rule otherwise they loose their uniqueness.