Maybe Mundus gets a kick out of doing it the sly way?
If he just came out and declared he was a demon and took humans by force, I'm betting he'd be pretty bored.
Then again, I'm not sure why he isn't bored anyway. He has humans under his control and they have no idea. He owns everything. It's like he has so much money and power and a sytem in place that he doesn't have to do anything.
It's like playing the sims gamte forever with all the cheats unlocked.
Maybe that's what Mundus thinks life is. Life is a Sims game for him.