Ah.. comes round once a year.
- Horse shit does not present the same parasite/pathogen risk that carnivore shit does.
- It is not practical to clear up there and then
- It is not safe to clear up there and then
- In some places, for example, a route a riding school take twice a day, people do come back and clear up a couple of times a week
Imagine having to hold a horse, and pick up sizeable shit, you can't carry a shovel really so you're changing gloves, picking up shit into a bag, changing gloves back again, then remounting horse whilst carrying a large bag of shit...
Thats really dangerous to do in a road. The rider has significantly less control on the ground holding the reins, whilst focusing on whats on the floor not on the horse or whats coming. That increases the chances of the horse getting loose, and a nasty accident and a horse through the windscreen of a car will absolutely risk driver/passenger death.
Riders should really get on using a mounting block, its harder and its bad for the horses back to get on from the ground. So depending on where the horse has crapped, the rider might be in for a long walk leading the horse back to a natural object they can use as a mounting block - and the whole time the horse is led rather than ridden, the risk to horse, rider and members of the public is higher.
Scraping it into the verge might be possible in some places/for some riders/horses but certainly not for all, you're still increasing risk whilst you do that, spending more time lingering in the road being a hazard. Local cyclists really won't thankyou for a verge full of slippery shit, its already a hazard with all the bits of metal, glass, plastic etc in there!