Culling from a breeding or training program doesn’t necessarily mean the same thing as say culling deer in the suburbs.
Just think about price. You have a well bred on paper horse that isn’t on a fast track to Grand Prix. He’s still going to be a better dressage horse for someone with lower ambitions than some other random breed. Likely still has more talent and big trot than your average OTTB or QH or Arab.
The Euro breeders can still sell these “pet quality” horses for a lot of money compared to slaughter sales. And buyers will want them, even to import them to North America.
It’s different if the horse is neurologically compromised or suffered a pasture accident young and can’t be ridden.
However, what’s being offloaded as healthy and sound but just not FEI quality from a structured European breeding program is going to be a lot higher quality than the low end QH and wildies and draft crosses that get culled at meat auctions in North America. Just like virtually any OTTB even one that flunked out of basic training is going to be the fastest horse in the barn even if they were a dud on the track.