Because the leg is so fragile and complicated that when broken, never heals and causes infections that travel throughout the body and the animal eventually dies anyway... it is the only humane thing to do...

A horse needs all 4 legs to be able to walk, so mostly it's because the break is so bad it can't be splinted or helped to heal properly in any way. A horse can't be put up in a sling to rest for too long. Their body weight doesn't allow for them to lay down easily for any length of time either. Their muscle mass makes it difficult for a sick or lame horse to right themself up and their organs suffer by beginning to shut down. Splints might work, and are tried at all cost. Usually though, a splint can only be used for a lower leg injury, or a less serious type of break. Before putting a horse down, an owner will try anything to fix it, especially if the horse is of a race horse that could still be used for stud to carry a good line.

They are not shot they are humanly euthanized, not all horses are euthanized, when a leg is broken, we have two in the rescue, one had broken his hip, is now fully ridable, and the other one had broken a bone in the lower leg. I am not sure what one it was, be he has since beed adopted and is a riding horse,

because the horse is in serious pain unless its not very serious its just inhumane to let it live in that agony

to clear a few things up. unless you are on a large operation, or if the horse goes down in the middle of a race track, the horse is not "humanely euthanized"... it is SHOT. the average person can not afford the euthanizations, nor the time. leaving the horse to suffer is worse that putting it out of it's misery. it could take the vet a week to get there... then what is humane? the medication is approximately $50.00 a miligram. you need 40 cc (mg) to appropriately euthanize a full grown horse. and no, not all horses are put down when they break a leg, many try to help them and get them the attention they need. but, in a lot of cases, it can not be aforded. horses are big, and their vet bills are big. with the current market, not many ranchers can afford to have the horse doctored, considering a horse can not be put under anesthesia, and the stress is likely to kill him before anything else does. we have had to "put down" five horses in the past two years, three of them yearlings. that seems like alot. but the yearlings sufferend from broken hind cannons when the floor fell out of the public haul trailer when they were being delivered to us. unfortunate. not bad odds considering we have over one hundred and twenty horses at our ranch year round.