Because when I look at humanity over the ages, I see a select few using technology and violence to exert power and influence over the great many to further their own desires.
I don't see that changing in the future anytime soon, especially before AI gets to be the next technology in a long line of technology used to oppress and inflict violence on others.
In China, in Tianenmen, they had to bring in forces from the countryside because the city police wouldn't fire on their neighbors. AI won't have any restrictions or morals. An AI-powered law enforcement force would gun down every single person in a city if authorities called for it.
Then the question becomes did those gun downed humans deserve what they got? And did that massacre result in a greater good? Those are the not-so-pleasant questions the humans will have to ask themselves (probably using AI to help, ironically).