In addition to the given answers, healing a horse's broken leg is usually combined with hanging the horse off the ceiling in a sling, so it is forced to rest its feet. This measurement would need to be done over roughly half a year, and many horses would pass away with a colic due to this. Also, amputation as mentioned below is questionable; horses live in groups and an amputated leg rather speaks against living in a group. There are some broken bones, that actually are fixable, but for most bones in the horse's leg, it's very hard even more so if have arthrosis on the 'healthy feet'.
The problem is, that the horse's metabolism is negatively effected and the horse experiences immense stress. Costs are also very high (starting from high 5-digits numbers [Euro, dollar], depending on the case), and as a result, many horse owners could simply not afford this treatment, which additionally does not promise good outcomes. I am not even talking about whether the horse could be 'ridden' again [I would not care too much for that], I only argument from the horse's perspective there.
Horses tend to solve problems with moving forward, even when in pain. As a result, they would even unnecessarily tear off their feet even if thinking hard and doing a cautious movement to free the limb would have left the horse unharmed. Do not underestimate the nature of a horse, this plays highly against treatment in the case of broken legs :/