I reckon its the 'economic repair' theory. Seeing the eyewatering costs my friends fork out for the dog or the cat, I expect the cost of repairing a horse will be proportionate to its greater size.
It's not 'just' economics - it's also mercy and practicality.
Due to a horses structure, psychology and physiology, a horse cannot be made to 'rest' or remain immobile, 'lying down', 'still' or even 'quiet' in order to permit satisfactory bone healing.
Generally, weightbearing needs to be avoided or severely restricted if a bone is to heal, but horses cannot be maintained in any sort of health for more than
maybe a couple of days, if they must lie down and be inactive. Horses even sleep standing up. Lying down is something they do do, of course, but only for relatively short periods - a couple of hours. Slings can be used to keep a horse upright as a leg injury heals (only some horses which have a calm disposition will tolerate this) but these bring problems of their own, the main one being that the hoof acts as a mechanical blood pump that works when a horse takes a step. They have no muscles in their lower leg or foot to aid the return of blood to the heart, and the hoof acts as a pump instead. Without being able to move about and stand on its non-injured legs, abscesses and laminitis (an excruciatingly painful condition) will develop. Horses are also very large animals and cannot benefit from the sort of 'help' and assistance which it is fairly easy to give a cat or a dog by an individual human. Purpose-built lifting and moving equipment and a team of people are needed for even the simplest procedure.
I don't know if anyone remembers the stallion Barbaro? This young stallion - a Kentucky Derby winner - 'broke down' a few strides into a race with a complex hind leg fracture; apparently he was also a 'real pet' and very cooperative and calm. Although lengthy and repeated surgeries, slings, pins, fibreglass casts and external braces resulted in the healing of his fracture - albeit with fused joints - laminitis and abscesses in both hind hooves resulted in his euthanasia some eight or nine months after his initial fracture.