I don’t think it is the ground so much as the breeding of the horses themselves. In the beginning of the Thoroughbred, they were meant for warhorses, needing stamina, speed, steadiness and soundness. They were the Hummers and ATVs of their day for purposes of war. To develop the best, kings would hold race meets to find the best type of horse over the course of a day, racing the same horses as many as six times. The best horse was bought by the king and sent to stud, free to be used for breeding by all the countrymen, on the say so that the king got first pick of the foals. Having to go to these things, the courtiers discovered a wonderful new sport, since you could bet on it. Over time, the soldiers needing horses became less armored, and therefore speed more important, especially with the invention of guns. A horse that could carry messages fast, or be a scout or run down fleeing enemies was more important than a solid beast that could carry a half armored dragoon. As horses became obsolete with the combustion engine, speed was all that the thoroughbred was wanted for, and that is all they have been bred for since. Not for soundness, or bone, or sense or temperament. If a horse is fast, he is bred to for more speed, nevermind that he can’t be handled by less than three people at a time. And in America it is worse, as our biggest races are aimed at babies, three year olds, their knees not even set. In Australia, their big race is the Melborne Cup, and it is twice as long or more than the Belmont. Their horses don’t break down near as much, perhaps some outcrossing is in order? Same for European horses, they race on grass, and often for longer distances, why not cross our horses more on theirs? Or, heretical thought, wait until they are five to race.