Because a human driver slows down in order to account for their reaction time and ability to continuously track multiple objects around them. Basically, they know that by the time they realize the kid is about to get run over, it will be too late to stop. Computers are already much better at tracking things and reacting to them with millisecond precision.

(That's not to mention the usual disregard for traffic rules - and thus basic safety - an average human driver has, which other commenters have touched on.)