I don't think it's people's beliefs that are the problem here (and agree with you that make-believe is fine). It's the implications of what a sentient AI would actually mean.

Many of us grew up with movies like Wargames and Terminator, so it's easy to see how people make the leap from 'is this machine sentient' to 'machines will take over the world'. That said, it's still a knee jerk reaction at this point - we don't have sentient AI, so don't have to worry about it eventually wiping out humankind.