Because it's him. He'd use extraordinary psychic powers (which has no reason to posess), to create a duplicate of himself, simply because of what? Vanity? There's no logic to it at all. There were other reasons I think, but it's really a point that needs explaining by both sides.
As for strong will, Sephiroth's mind is weaker than most average people by that point. Jenova is one of the most powerful beings ever.
Jenova's body wouldn't be conciousless, given her arm is capable of independant action.
