It wouldnt be hard to weigh each horse as they went to the paddock, but i would imagine that this information isnt info that bettors, owners, trainers, and jockeys are clammoring for, so, its not info that gets published. Dog racing has the dog weights available, im not sure why dog weights matter more than horse weights matter, but the dog industry seems to have no problem publishing their weights.
When a paddock reporter says a horse has lost weight and its visually noticible to them i would imagine its about 50 lbs.
For me personally, if a horse is underweight and not carrying good flesh, i\'ll be able to notice it and proceed accordingly even if i dont actually have a number.