* They are bred so much for speed -- too fine of bone to support the work demanded of them while they are still babies. European and S. American and Canadian tbs generally have significantly more bone and substance than US-bred.
* TB knee joints don't close until they are 4 years old, yet they are raced at 2 and 3 years old. Think of load up a young child with weights and forcing them to run laps on a hard track day after day.
* Back in the day, they weren't raced nearly as often now, with 2-4 weeks between races and seasonal racing. Most of them are horribly over-raced now, weekly and year round, shipped south for the winter circuit.
* In the interest of speed, track surfaces have been made harder, increasing the concussion on their legs and hooves.
Many, many thoroughbreds die each year as a direct result of injury from racing. Their feet tend to be a mess, due to shoeing at too young an age and bad trimming. Thousands end up permanently unsound and unfit for other work, and many if not most of those are sold to slaughter.
Their unnatural, high-stress living environment leaves them very susceptible to ulcers and psychological damage that manifests as "weaving," "cribbing" (aka windsucking), "stall walking" and other repetitive motions that suggest mental illness.
I am a lifelong horse woman. Horse racing has contributed much to advances in veterinary care and knowledge, which benefits many horses benefit from. Without the money horse-racing provides, a lot of research may never have happened.
However, I am NOT a fan of horse-racing in the US. I cannot stomach the thought of humans being entertained by what amounts to slow torture of any animal, let alone a horse.