imagine the following:
if someone weighing 60 kg climbs on your body and you have to carry that person a few meters, let's say you carry that 60 kg person for a distance of 30 meters, would you be able to do this every day and hours? I don't think you could, so how would an animal like a horse not be too much of a strain on the animal by submitting such a person to it? The problem is that we are normalizing something cruel, athletes are not carrying weight, I would agree if horses just raced without them having to carry weight. but people don't want to do that, they want to enslave the animal in such an inhumane way and they are happy with that attitude
I have to admit that I was surprised and laughed a bit to see this kind of example.
Can't believe you compare our capability to horse capability in terms of carrying weight.
If we are 60kg, horses can't even feel our weight carrying at a distance of 30 meters. That was too easy for these horses as their body is naturally stronger than us. To add to that, these horses are carefully taken care of with lots of supplements and vitamins making them more strong. Do you think farm horses are the ones used in horse racing? Regardless, nothing will change the science behind it if we take your example.
In horse racing too, there is a weight limit for horse jockeys. As an owner, how can you make money on your horse if you put big loads on your horse?
There's always a weigh-in before the race to ensure that horses can do a proper race.
And just for your information, racing horses are not working every hour. In my knowledge, it's only twice a day they are put into the race because obviously, why the owner will put their horses in heavy loads? These owners and operators are not dumb to put their horse in a situation like that since that's their investment. How can they make money in return if they will always tire their horses? Think, mate.
to gambling. 
