I'm going to bounce off of the helmet topic, and ask: why don't people learn how to fall?
Yes, helmets do lessen the risk of head injury. We get that. But it's not like riders only die from head injuries. Plenty of people break backs and bones from falling off and almost all of those injuries could have been lessened or prevented had the rider ever learned to fall properly.
Martial arts and gymnastics focus on teaching the student how to fall without breaking limbs. Why don't horseback riders learn the same? I can't tell you how many times I've saved myself from injury because I've been able to apply that knowledge. It's so ingrained that I instinctively know how I have to react in order to reduce injury every time I fall.
Personally, I rarely use a helmet. It's so much easier to maneuver while falling without it, and I don't get injured by trying to roll with a helmet on.
If people are really so concerned about their safety that they refuse to get on without strapping a helmet on, shouldn't they want to be proactive and learn how to fall properly as well? The helmet might protect against knocks, but it's not going to do anything about the broken arm you got from stretching out to slow your fall, or the broken back you got because you didn't tuck before hitting the ground.