Why do people AI doomsay?

What is the logic behind AI being the end of human civilization, doomed to rapidly bring widespread destruction and untold amounts of suffering?

People say this, and then just refuse to elaborate. They just say "We can't control it!"

Control... what? What specifically is the threat?
When a fastfood drive-through AI takes my order, is the ice cream machine plotting to start nuclear armageddon? Is it developing consciousness through sheer randomness like a Boltzmann brain and hacking into the "network"? …

artificial civilization control destruction drive human logic people through