Parts of our bodies have excellent self-repairing facilities. Parts. Your liver won't repair itself if you damage it enough. You have one set of lungs, so if you smoke enough they most likely aren't growing back. And most critically for this discussion, ligaments and cartilage will not heal once damaged. They are optimized to be as tough as possible, at the expense of not healing at all once damaged.

As the recipient of 2 knee surgeries, and possibly an upcoming 3rd, believe me when I say that I would like nothing more than to have my knees "self-repair". But it just isn't going to happen naturally. You get one set of cartilage and ligaments, and once they are gone, they're gone.