Why ? Obviously—well, it's obvious to me, at least—there's some kind of barrier between the material world and the other planes. No, let's call it something pseudo-mystical—say there's a curtain between this world and the others, or maybe a veil. Okay, some kind of curtain. Sure, that made sense, otherwise people might just stumble from this world into some freaky metaplane without intending to do so, or even knowing it happened.