Horses are, for want of a better description, weight carrying animals but we deliberately minimise the amount of weight they carry in order to allow them to run faster
Not sure why people are fixating on weight all of a sudden when it's hardly the main issue, but if you must, then consider this. Top weight in the GN is generally just shy of 12 stone, so around 75kg. Since few jockeys weigh that much the difference is made up with lead, which isn't the best thing for a horse to carry, as it's dead weight. However, at around 15% of a race horse's bodyweight it's acceptable, and lower than the 20% cap other governing bodies use.
Sure, it's less than many leisure horses carry but A there are concerns about what leisure horses carry too and B riding club riders don't gallop 4 miles + in 10 minutes at an average of 29 mph whilst negotiating some 30 large obstacles.
Horses have not evolved to carry weight. They are not weight carrying animals and the structure of their backs is such that actually, weight wise, you're better off on either a camel or a cow.
Saddles are designed to lift and distribute the rider's weight away from the spinosus processes and delicate nerve endings. If horses were so natural at weight carrying, would we need saddles designed like this? One way of assessing when horses were first ridden is to examine ancient skeletons for signs of back damage - certain types of back damage only come about through being ridden.
And then there's the myriad of back problems ridden horses develop, such as kissing spines.
Despite all this, it's not the weight carrying aspect of horse racing that bothers me, but if you must bring it up, at least be accurate about it.