Yes it is sometimes possible to repair a broken leg, however normaly when they brake a leg in the national it is generaly a catastrophic shattering of the leg.
If they havent shattered immediatly then the horse keeps trying to get up and follow the pack and normaly end up with a compound fracture.
Niether of those are repairable and often even in humans result in the loss of the leg or severe disabilities afterwards
Also having known a mare who had to have her leg pinned after a minor fracture, the pain and distress that mare went through just so that she could be kept alive and bred from was horrible to watch and thee is always the worry that she could canter round the field one day and it give again.
I personaly would never put a horse thorugh the pinning process
Also have you ever tried to completely immobilise a hose for months on end?
I've had to box rest a horse for 6 months and that was horrific, I've also had to care for a horse who was in a sling for 2 weeks and the terror that she would struggle and hurt herself, or get pnumonia kept me awake at night. She had to be sedated for much of those 2 weeks, not fun, very expensive and not something I would voluntarily put a horse through myself (I was working as a groom for a very wealthy lady)