Because if you woke up as me, you'd be me. Let's assume that consciousness works in roughly the way implied by your question: it exists, but it exists entirely

Because if you woke up as me, you'd be me. Let's assume that consciousness works in roughly the way implied by your question: it exists, but it exists entirely separately from both the body and from mental contents. Now, let's further assume that the scenario you're proposing actually happened. Last night, in our sleep, you and I switched souls. Yesterday's me is actually you now, and yesterday's you is actually me now. How can either of us tell the difference? Clearly the contents of our consciousness - our memories, knowledge, skills, perceptions, emotions, etc. - are attached to the body, particularly the brain. This is just the assumption we started with. What, then, makes you you and me me? Clearly our souls aren't doing a very good job at telling the difference!