A couch is soft and comfy. Apart from the bed (don't even get me started on that one), it's quite possibly the most comfortable place to sit in a home. But if you're a dog, you're likely to be treated like a criminal if you're caught sitting on it.
You'd think people would want their dogs to be comfortable. Instead, most of us are relegated to lumpy dog beds. Why should a dog have to sleep on the floor when there's a perfectly good couch nearby?
Thank goodness I live with Master W. He treats me the way a dog should be treated: like a best friend.