They absolutely are NOT bred to have small bones, quite the opposite. If there is a complaint that makes sense, it's that the horses are too young at two, or three and their bones are not yet mature. If they waited till three to begin, and didn't race them hard so young, fewer would break down. If you said that, you'd be right. But what you said above was not correct.
Humans make the choice to be athletes, and injuries to us don't often result in death. I love horses. What else can I say?
I understand you love horses, I do too. I wanted to hopefully impress on you that people who keep horses do so in order to engage in horse sport, be it racing, or jumping, or rodeo, or trail riding. All have dangers to both horse and rider. But I would argue that life has never been better for the horse. As valuable and pampered athletes, they get the best care in the world. As wild horses, they had short lives before becoming hurt, starved or eaten. Work horses, they got a bullet to the head, not expensive medical care. Yes, horses do sometimes break down in sport. But even those who do break down have it better now than they ever did before.
Just wanted to correct a couple misperceptions, dont know if it'll make you like racing, but at least you won't be wrong about what you don't like. :~)