Lore reasons. In the WHM storyline you encounter a would-be necromancer. The products and byproducts of necromancy poison the lifestream and the effects are felt miles away by the Gridanian elementals. Being a necromancer as a player is about as shady as trying to justify FCs summoning primals. Doesn't make much sense in the lore scheme for the Warrior of Light to walk around poisoning the land and disrespecting the dead.