In the X-Men 2 (2003) Magneto escapes from his plastic prison by extracting the metal that Mystique injected into a security guard.
Why was this necessary? Why can he not control all humans? Let me explain.
In one of the episodes of the Avatar: The Last Airbender animated series, the gang ends up in a swamp like place where they meet a woman who can waterbend. But not only that, she can also manipulate things made of water, like plants and if I recall correctly, people.
Anyway, the human body contains metals such as copper, magnesium, manganese, zinc, iron and even aluminum and mercury, which are metals present in the human body.
So, why is it that Magneto cannot do what the waterbender was doing and take advantage of the fact that human bodies contain materials he can manipulate?
Update: as stated in X-Men Wiki article about Magneto, he CAN in fact manipulate at a sub atomic level
Matter Manipulation: Magneto is able to manipulate matter even to the subatomic level for a number of purposes. This might explain why he is not limited to objects with metallic properties and is able to manipulate objects with non-metallic properties like wood, stone, plastics, etc. to such a degree. This is due to the electromagnetic force which exits between atoms and other subatomic particles.