No horse has to be sent to the knackery. However, most of them are as they are considered more of a commodity or investment, which is rid of when no longer profitable. Most jumps horses will end up at slaughter when they are no longer commercially viable or profitable.
Jumps horses are profoundly hard to rehome, as they will often sustain injuries in the sport. Therefore, they are a lot less appealing in comparison to flat racers, and often impossible to rehabilitate.