[lucubratory]

But now we don't need horses so there are far fewer of them, and they are so completely unimportant to the economy that if they caused us any significant inconvenience we could kill them all without it being a significant issue.

From our perspective the lives of horses look pretty horrible, particularly their lives in service to us, but I don't think it would be very comforting to have an AI explain to me that humans aren't fully aware of the world in the way that greater intelligences are, that because of the mental issues in humanity which cause them to do annoying things like bombing data centres or cutting fibre optics cables the human population needs to be significantly resized to a more sustainable number, or anything else a human-equivalent that actually cared might explain to a horse-equivalent that could actually listen.

The point is that horses don't control their own destiny, and they aren't even important enough for us to particularly care about said destiny in anything other than vagaries. Horses that interfere with human society in some way get put down. I don't want to be put down or have no say in the future, so we shouldn't attempt to build something which has a reasonable likelihood of being as much smarter than us as we are smarter than horses.