According the this site:
https://www.horsedeathwatch.com/background.php they have seen more than 2000 deaths in around 5500 days. Most of the horses die during the match as well, they also have instances where they are exploited using drugs (doping).
It's pathetic to know this fact.
But, in fact, Horse Racing has always become one of the most favorite bettings in the world, moreover many big gamblers from rich people love doing betting in this Horse racing. So, this may be very hard to stop or ban this betting. You know, big business is there and we may fight with many rich people with high powers in the world, or in each country.
Those deaths are logical enough. The horse may be also treated as a human athlete who must also do some diet that will make their body healthy and fit in every race, high periods of training every day, and other pressures that of course got by the horses.
I think this is why so many people have problems with this, after all horse racing is very popular at a great deal of countries around the world, also we know that a lot of money is moving in that industry which could allow to treat racehorses as they should, because if it wasn't for racehorses then the sport will not exist at all, and yet the mistreating of racehorses continues so this is not a matter of money this is about the fact that many owners do not really care and believe that they can do whatever they want with them.