It certainly *can* be fixed. Just like that of the vast majority of all vertebrate limbs.
(image: Equus)
Unfortunately for horses, a broken leg almost invariably means an end to their “career” if they are a racehorse or any type of working animal. This is due to the time required for the bone to mend and also the – probably correct – perception that it will never be as good as an unbroken leg and the down time resulting in a huge loss of fitness, plus the associated tissue / muscle damage.
Another issue that compounds the problem is that breaks in a horse’s leg often – owing to their body mass compared with limb bone strength – involve the bone shattering into multiple pieces, causing further damage and making surgery difficult and hugely expensive.
Its just not a viable thing in the eyes of the owners. There are some charity / rescue organisations who will take on some such animals but obviously they can’t take every one along with the associated expense.