"They're bred for it" and "they love to do it" are bullshit arguments. Horses participation in racing is the result of induced compliance. The environment of the track, riding equipment such as bits, bridles, etc, and the use of whips all exist as a means of forcing the horse to run. They are not there as autonomous competitors and they're not racing by choice. Everything about racing is designed to override the natural behaviours of the horse and push them into doing what humans want then to do. Even breeding is done by force and with suboptimal welfare at its heart. Dubious practices include the use of "teaser's", usually horses not suitable for breeding, they're put in with a mare to see how she reacts in order to check if she's in heat however this often results in injuries to the teaser as she is liable to bite/kick him. The prevailing attitude is that it's better she kicks the teaser than the expensive stallion. In-demand stallions are flown around the world (at incredible stress to the animal) and mares are restrained for forcible breeding. The thoroughbred gene pool is incredibly narrow, and getting narrower all the time, which results in a lack of genetic diversity and increases the likelihood of producing horses with defects, propensity to injury, and physiological weaknesses (all of which increase their risk when racing).
"They live like kings," except they don't. Not at all. Thoroughbreds are an expensive commodity and are treated as such. Rollicking around in a paddock brings with it the chance of injury. The horse might eat something it shouldn't in a field of grass. If it's with other horse they might hurt each other. They could be stolen. They could fall. They could get too hot or too cold or - heaven forbid - they might do all manner of normal horse stuff. The majority are kept in enclosed, highly controlled spaces and only get out for a very restricted period each day before being enclosed once again. As a PP pointed out, hoof problems are rife as are ulcers and it's from being stabled for the majority of the day. They don't eat like kings either, they're fed manufactured food designed with performance in mind but it isn't good for them and, by the age of three, over 80% of thoroughbreds have chronic gastric ulcers. By the age of three, over 70% will have suffered at least one significant injury.
And that's the horses who are the chosen few, the ones who can win. Let's not even get into the issue of wastage within the industry.
All the stuff about the love of horses and the love of the "sport" is crap. No one who cares about horses could put them through the exploitative horseracing industry where winning is placed at a higher level of importance than welfare. None of it is for the benefit of the horses.
The whole point of steeplechasing is the risk. It's no coincidence at all that it's popularity took off at the same time as dog-fighting and bear-baiting were banned, horseracing is essentially a socially acceptable bloodsport. As unpalatable as it is for racing fans to admit, people watch it in part for the possibility of seeing falls. And that's not just hyperbole, studies have shown that the punters want it to be risky, they enjoy it more because it makes it more exciting. The fences are designed to push the horses to their physical limits and test their survivability rather than their skill. Statistically, most horses fall at the first fence on any given steeplechase but the odds of finishing a race diminish with each subsequent fence because the risk is accumulative.
I'd also like to add that the carbon footprint and degree of water consumption in horseracing is staggeringly huge too and is contributing to climate change. It is also explorative to people lower down the hierarchy than the elite owners. Lads (especially female Lads) work in shockingly bad conditions for minimal pay and are treated appallingly. Jockeys are also exploited, eating disorders, alcoholism, and drug-use pose significant a problem in the industry. There have been many jockey deaths and life changing injuries over the years (not as many as horses though).
But still, at least you can have a few moments of happiness betting on the gee-gees while funding an exploitative industry built on the literal dead bodies of horses and their riders.