Because it, and the entire industry around it, generates ENORMOUS amounts of money for almost all involved (typically not the average owner, most jockeys, or stable lads/lasses mind).
It's never getting banned, and horses absolutely will die in horrific ways doing all sorts of other things including wandering around a field minding their own business.
What could be achieved if the right campaigns were supported, is altering the age at which horses can start training and start racing - up it by 2 years. That would instantly cut the number of horses bred because thats 2 more years of them taking up space and eating and generating vet bills for their breeders - and cut the number getting injured as they'd have 4 or 5 years to mature not barely 2 or 3 (horses enter training at their 2nd birthday for flat racers, 3rd for jumpers but many jumpers are failed flat racers who have already been in training a year), which is the 1st january following the year of their birth meaning they are barely beyond foals in some cases).
Horses are not actually done maturing skeletally until 6 or 7 years old, so racing 2 and 3 year olds does horrendous damage to them which doesn't tend to be too obvious unless they break down on the track, it will show up when they hit five or six!
It is telling that those bred to jump, who start a bit later, do carry on into their early teens, whereas those bred for the flat rarely are still in racing by 10.