I've been wanting to make that point, also. Chris Reeve had only been riding for a couple of years and was doing something I wouldn't do with 35 years riding experience. Jumping cross country is DANGEROUS, Chris had lots of money to spend on horses that were out of his league and probably had trainers telling him how great he was and was pushing him to do more than he should. Jumping is hard and shouldn't be attempted by someone who learns to ride as an adult. You get a better feel for it when you start riding at a very young age, imo. Like gymnastics.
I didn't know he had only been riding for a couple years. That makes jumping even more dangerous --- it isn't a sport for beginners.
But you have to go cautiously, especially in cross country, because unlike the jumps in the ring those obstacles don't fall down when you hit them. I have no illusions about my ability - I'm a competent amateur - and I have a bomb-proof, reliable mare who is slow-and-steady rather than a top fuel dragster. We don't go out trying to set any speed records, and we compete one level below what I think we could do if pressed.
Having the money to afford Too Much Horse is probably the single most dangerous factor - the worst riding injury I ever saw was directly due to that. But it's also the accumulation of mistakes - attempting too much too fast, riding above your ability, and then making a mistake of judgment or inattention while on course. From what I hear from horse people, all those things were a factor in Mr. Reeve's injury. But I've never heard a complete account of exactly what happened from a first hand source.