Because then suddenly it's a tragical accident caused by a software fault, with no human responsible - just as most data breaches are framed as "cyber attacks" instead of what they actually are: negligence.

Horses need to be trained (usually they don't like trampling people) and have high maintenance cost. As for the wheeled vehicle, it can be prevented access with roadblocks. Also it can't enter houses. Something that can walk over piles of rubble would be clearly very useful.

Because you need a human to control it. A robot however could just be sent on patrol and its AI would do the rest leaving no one to blame.

Because it's utterly terrifying?