City streets were intensely filthy back in the days of horses.

They require a constant input of hay.

They're "self driving" in the worst possible way - they can run off on their own and do whatever they want, and have little understanding of the rules of the road. People already freak out when a robotaxi takes an inadvertent wrong turn, horses can freak out and try to kill pedestrians.

They're slow. They're hard to manage. If you don't want to be exposed to the elements then you'll have to build carriages, so you'll still have factories and whatnot. Horses eventually just up and die regardless of how well you care for them.

Horses are not better than cars.

It isn't mentioned much but there was often a 3 foot high lane of manure down most city boulevards