system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is euthanasia often the only option when a horse breaks its leg?
gpt
Because a horse needs to stand up and move to be healthy.  Horses do not sit down.  They often sleep standing up.

They aren't designed to be off their feet for extended periods of time.

A horse will never stay still long enough for a leg to heal properly. They are just massive animals, whose bones are massive, but fundamentally little different than yours or mine,, and can only heal slowly.

If the break is bad enough, then there is just no way to keep the animal from being in constant torture.  It will put weight on the broken leg. Any healing that may have occurred will be undone.  The horse will be in incredible pain.  Repeat daily until the horse is driven mad.

It has everything to do with compassion for the animal.

It has very little to do with cost. If it were possible to immobilizer a horse long enough for a fracture to heal, they would have figured it out centuries ago.  But there isn't. 

These are not 100lb dogs.  These are 600-800lb horses. [Fixed typos]