Because there are risk-takers and risk avoiders in this life. Many people learn and become good riders in adulthood, and compete in equestrian sports. There are beginner levels in jumping, and they are full of adult riders. Any rider, experienced or not, can have a fall. Most don't get injured to the degree he was, but the possibility is always there. That possibility keeps some of us on the safe side, but the winners took the risk.
You are right, of course. Still, it makes me cringe to see an adult who hasn't been riding very long take risks and you can't tell them anything, either. They don't know how bad of an accident they can have and how quickly it can happen, unless you've had one. In the blink of an eye, you can be on the ground with a horse on top of you and not even realize anything was going wrong. I've been there, done it several times, had horrible accidents, but I was a kid and very stupid. I wasn't always able to walk away but when I was able, I always got back on. That childhood experience taught me ALOT.