system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:why are horses legs not repairable?
gpt
Horses can't lay down for long periods of time, it's bad for them.  If they're not sleeping, they're standing up, and prolonged laying causes all sorts of problems.  Given how long it takes to heal broken bones, sedating them for that long isn't an option.

Horses are also generally a nervous, skittering animal that want to be free to move around.  It's very difficult to get them to stand *still*, and when you try to force them to it makes them even more nervous, which makes them more likely to get jittery, which makes it more likely they'll stomp with the leg that's broken, which would just make the leg worse.  Given how heavy horses are a cast just isn't going to cut it, they have to stay off the leg.  But you can't lay them down and you can't hold them still in a sling, so that's pretty much all the options gone.

For all of that, their bones just don't heal well, and given the nature of what horses *do* (gallop around at 30 mph) even a slightly-less-than-perfect fix to their bone can easily break again.  Bone breaks on any animal aren't guaranteed to heal perfectly, but on a dog or a cat you've only got a few tens of pounds of animal to hold up, and if they end up not being able to run and jump like they used to that's mostly ok, they can settle down and be lazy.  Horses can't, they're big animals and running is their thing.

Attempts have been made to repair horses' broken bones, but it universally does not end with a positive quality of life for the horse.  At best you'll get a horse that can walk, mostly, with a decent amount of pain, and the trauma and pain of setting and healing the bone is often too much for the horse.  Given the lack of success we've had for the literal millennia we've been riding horses, the most humane thing to do is euthanize the horse so that it doesn't suffer.