Injury mainly. Sickness too. Have a look at a horses legs compared to yours. They got 4 of course but they carrying 500+kg. Theres websites showing bone structure etc of horses leg joints. Compare them to yours. Doesn't take a lot to go wrong to stop a horse. Or slow it down even. Theres not a huge difference time wise between the good ones and the not so good ones. Good one gets an injury they back in the field pretty quick smart. All those you mentioned had problems.