Real humans don't behave like that. They behave in all sorts of weird ways when they know that they're interacting with a machine. They SHOUT, O-VER-E-NUN-CI-ATE and use unnatural sentence fragments, which craters the accuracy rate of a speech parsing algorithm that was trained on natural speech. They get instantly frustrated, because they assume that this machine will be just as stupid as the archaic IVR system that their bank uses. They assume it's a scam robocall or a prank and hang up without saying anything.

Acting like a human is by far the most efficient option for a system that is designed to do what Duplex does.