This website explains some of the excellent welfare and safety initiatives taking place in British horse-racing currently
www.racingexplained.co.uk/horse-welfare/the-horse-comes-first/
"In recent years the average number of runners per annum is in excess of 90,000. Within the last 20 years, the equine fatality rate in British Racing has fallen by one-third (from 0.3% to 0.2% of runners)."
And I agree with others on here that this figure can still be improved upon.
Horses are prey animals. They are designed by nature to run in short bursts of speed as far as necessary to escape a predator; so the sport of horse-racing has built on that natural tendency as has the breeding of the thoroughbred over hundreds of years (a sort of speeded up evolution).
As a previous poster said, some areas of the horses conformation make it fragile (ie during the gallop at various points, all the weight of the animal is resting on one part of the lower leg and hoof). And if you keep or have kept horses you know that they can injure themselves just as easily out at pasture, as at full gallop.
From website above:
"A study by Liverpool University found that 62% of “traumatic injuries” (ranging from grazes to fractures) suffered by a sample of leisure and competition horses occurred when turned out in the field, compared to only 13% during ridden exercise. The British Horse Society also estimates that there are over 3,000 road accidents annually involving horses."
And if you've visited horses in a virtual wild or natural state (in Argentina for example) you can find them in a pretty sorry state: starving in some cases, in agony with terrible lameness or leg injuries, or skin diseases. So don't assume it's all tea and cakes for horses just 'loafing around' in the wild either.