> I really can't understand how people seem to expect "intelligence" to be deterministic and infallible

We don't really expect this of intelligence

We do sort of expect this of machines

Most people in society use machines and computers expecting very deterministic behavior, within some constraints. A saw cuts wood as long as your blade isn't dull. A computer shows your emails as long as the power and Internet is not out.

Comparably Unconstrained computer AI systems are still extremely new and there is no social norms or expectations around it yet