then why cant the player do it?
Because the gameplay doesnt allow it. There are always things that should be available to the player according to lore, but are not because of balancing or game engine limitaitons. For instance, theres a wizard in Morrowind who summoned a dozen scamps for manual labor when uncovering hidden areas underground. If the player could summon an army of permanent daedra, it would strain the game engine, and it would probably make things unbalanced too. But according to the lore side of the story, you would be able to if youre powerful enough.
Edit: Bah, ThatOneGuy beat me to it. By far.