Yes everybody is right. Eventing started as a way of choosing the best horses for the calvary. Dressage a test of trainability,x-country to test stamina and boldness and stadium jumping to test accuracy and soundness as it is always run last.
I ride both Dressage and eventing and don't understand the multiple test either. I think its an Ego thing.
Injury rate,who's knees ? The horses are extremely well managed and have a competitive carreer into their late teens. The riders- we won't go there.