The same reason there are timed rounds, gloves to protect the hands, mouthpieces, etc.. IMO, it has more to do with organized crime, specifically gambling, and the need to make an event as equal as possible so you can have people gambling on both sides. That's why they give odds - give good enough odds and someone will bet on a fighter they typically wouldn't bet on. Same reason they cooked up point spreads in football. From a "business" view you can't have 80 or 90% of people betting on a fighter and only 10-20% betting on the other. You'll lose big money that way.
I have seen some martial arts "games" in Korea and China in which there were three rules: No attack to the eyes, no attack to the windpipe, and no attack to the spinal column. I never saw the rules actually enforced but they did exist. The games usually lasted less than 40 seconds and had no weight classes. I've never seen them in the US, probably for liability reasons.
"The same reason there are timed rounds, gloves to protect the hands, mouthpieces, etc.. "
My response, which amplifies yours, is that ALL SPORTS COMPETITIONS ARE CONTRIVED. The rules are made to establish the boundaries within which the competition takes place. This should be obvious, but in the "era of instant replay" we sometimes forget.