Magic is a primeval force, the one true elemental force in the world. And it is stored in human beings.
The most powerful wizards are the ones that have the most mana-slaves. The most powerful wizards fight amongst themselves, fight for prestige, for power, and for slaves. In a world of CAVEMEN WIZARD LORDS, only one Wizard Lord can reign supreme!
This entire idea came from a friend talking about "what if people could be mana batteries", and "what if magic has always been there?" And now we know what'd happen. Who would have time for science or for development of agriculture if the most powerful resource in the world is people themselves?