I personally agree with all of this. I agree that our body wants to heal, and that healing happens by the body without any external inputs.
The problem, however, is that we cannot always recognise the signals from our body. Sometimes the message gets distorted or overridden - by our mind, our subconscious, or our ego. Sometimes, we hear the message but we misinterpret its meaning.
On a purely physical level, our modem environment makes it difficult for us to understand the message. When we spend years ignoring our natural instincts, is it surprising that we might have lost touch with our true self and our true needs? We rob ourselves of sleep, night after night, and use food or drugs or stimulating activities (like caffeine and sugar and television) to prop ourselves awake, without repaying our sleep debt. We sit all day in dehydrating office environments, ingest dehydrating substances (like salt and caffeine and alcohol), without ever properly replenishing our fluids. We use food or drugs to numb ourselves from the realities of our life.
So while it seems simple and beautiful to just "listen to our bodies", unfortunately the message we receive is often far from the truth.