This is purely conjecture, but I would guess that the reason sudden deaths appear to be more common during the cross country phase of competition than during schooling, turnout, or other horse sports is not due to eventers being uncaring people who see horses as disposable, but due to a combination of factors surrounding the nature of the sport and the demographics of the horses competing in it.
Horses aren’t prone to atherosclerotic heart disease (the primary cause of human “heart attacks”) the way humans are, but I’m sure as they age their vessels likely still become less compliant and “elastic” … and therefore, like an old rubber band, more brittle and prone to catastrophic failure under stress. Upper-level event horses are often in their mid-to-late teens, and while that isn’t ancient by any means, it’s old enough that in many horses the heart and blood vessels may be less resilient to extreme stress … compared to racehorses, for example, whose heart and blood vessels are also under stress during the major exertion of a race, but are usually much younger than event horses (particularly those at the higher levels).
The length of cross country, compared to most races, showjumping rounds, or gallops in the pasture, means that over the duration of the course the heart and vessels are exposed to that amount of stress for a much longer period of time, probably further contributing to any increased likelihood of major cardiovascular events.
Hopefully, the more we learn about the factors that contribute to sudden death on course, the better chance we’ll have of being able to detect horses at risk. But the nature of the sport and the the horses that are competing in it means that equine participants will probably always have a higher likelihood of dying suddenly on course compared to, say, show hunters or trail horses … the same way a human competing in a triathlon is more likely to have a cardiac event than a healthy person doing a 4-minute long figure skating routine.