They don't inhabit all realities simultaneously. The Nether is connected to them, but it is a separate dimension. Demons have to forcibly claw their way into a reality, or find a place where the Nether breaches it. Demons also perceive linear time just as mortals do, even in the Twisting Nether. Demons talk about time in a very conventional sense:
- "After ten thousand years of waiting, he is prepared to invade the world of Azeroth once again." (WC3)
- "This is almost too easy! If I'd known that this mortal resistance would be so weak, I would have launched this invasion centuries ago!" (WC3)