Considering that clerics have NO literary source, how do you justify their existence in the game? They are purely a construct of the game and are only present because someone needs to heal the party. Yet, they've been part of the game for a very long time.
This is exactly my point. D&D has drawn on whatever works. There has never been any sort of "purist" concept to the game. It's always been a kitchen sink approach with whatever developer grabbing whatever idea and stuffing it into the game. Many of the elements you seem to be attributing to fantasy are nothing of the sort.
Let's see, Monster Manual elements with little or no fantasy link (note, I'm doing this from memory):
- Beholders
- Mind Flayers
- Githyanki
- The Blood War
- Rust Monsters
- Umber Hulks
- Most Golems
Just to name a handful. You seem fixated on maintaining some sort of purity that never existed in the game in the first place. Classes, races, monsters, spells - all elements drawn from non-fantasy sources.