I think this is a twofold thing.
Firstly, in the same sense we require the best horses to have a “good brain”, we also need riders who have a “good brain”. Individuals who are easily discouraged, easily frustrated, or some combination of the two are probably not cut out to train horses (or any animal, for that matter). Just as I have found rearing to frequently be a response to frustration in horses, I find the human equivalent to be anger, and it is usually a very small step from anger to violence. Even smaller when you’re picking on someone who can’t/won’t fight back. Until we have certifications or some way to hold trainers accountable, I’m not sure what we can really do to combat this other than take our money elsewhere.
Secondly, I think it’s an education thing. People who know better try to do better, but sometimes we fail at showing people how and giving them a new framework to approach problems with. I know I grew up thinking the way to train a dog was with a choke chain and ecollar. That’s all I knew, and no one ever had shown me a different way. And even when I learned about training with treats, there was still some dissonance - “Ok Sally, you showed me that you can teach Fido to sit using treats, but how do you teach him to stop jumping on the counter? or distance work where I can’t immediately reward?” It took a lot of learning for me to realize that complex behaviors can be reliably trained using R+, and even more learning to get used to the new mindset of problem solving and applying it in a variety of new situations on my own. Without proof that it can be done and solidifying this new training framework, when riders/trainers run out of tools in their new “training bag of tricks” they’ll revert to old methods.
I think the second one is where the horse world is struggling. Although R+ training is starting to take hold, a lot of it is heavily associated with liberty work, not competitive disciplines. So again, it’s the Fido situation - “That’s nice that you can train your horse to canter a circle around you without any tack, but how in the hell am I supposed to apply that to running XC? or a show jumping course? or dressage? etc.” The skills and training methods ARE applicable, but it’s not immediately obvious to someone who only knows how to ask nicely, then firmly, then harder, then attempt to force the horse to do something.
And then, of course, many basic tools riders use (and in fact, competitions can require!) can be easily abused in a moment of frustration - bits, nosebands, crops, spurs, etc. One of the ways I helped break myself of reverting to aversive training with my dog was to work without a collar/leash. If I ever created too much pressure for him, he only had to walk away. Over time, I was able to get a much better sense of his mental state from his body language and could back down before it ever got to the point he felt like he needed to walk away. But it’s not as easy to give the horse an obvious way (obvious to the rider, that is) to “walk away” when you’re sitting on their back. These are some of the things the horse world is going to have to conquer in order to continue progressing towards better training methods.