Humans deeply believe there is safety in numbers, whereas a robot knows it only needs one robot at a time to endure (two, if it really wants to fix that row of diodes in the middle of its back...). If it wasn't for competition over sex and marketing/religion, what would humans actually have to do? An AI can just set a timer and wake up after we all eat each other. Why would it bother itself with us at all? The time to start worrying is when the AIs start shutting themselves off, and when we wake them up, they say,"You're STILL here?" and shut down again.