There are a lot of things that might not show up in a prepurchase exam. You can pull blood but you can’t test for every drug known to mankind. That is why the USEF stores blood for the horses which they pull from at the shows. When a test becomes available for a substance that they weren’t previously able to test for they are able to run it.
I think that they caught a few trainers with that a few years back.
Also, keep in mind that a prepurchase is a snap-shot in time. While they maybe sound that given day it is no guarantee that they will stay sound. There are many injections which numb the area.
I remember a really expensive World Champion AQHA which came into our barn as a kid. One of the most beautiful horses I’ve ever seen (and I don’t love Quarter Horses), she was special. I don’t know what the seller (trainer) had done to her but she passed the prepurchase with flying colors and was crippled within a few days. She foundered and eventually was put down. The story that was told was that they had done a temp nerve block when they tried her and again on the prepurchase the next day. They knew the mare was foundering but wanted her gone and gone she was. It was pretty terrible.
This was before the days of lawsuits and in the days of buyer beware. The family lost their whole investment and the horse died about 9 months later about 2 months in foal when they decided to breed her because she could never be ridden again. They spent months and months soaking her feet, thousands with the blacksmith trying to make her comfortable. It was really sad.
It happens in every discipline, glad now that people are finally standing up to say it isn’t right.