I think you're calling us morons really....
Well maybe or maybe not. However, it has been proven that in an emergency evacuation, many passenegers have been unable to get out of their seats because in their panic to get out, they cannot open their seatbelts because they revert to using the muscle memory for undoing their car seatbelts, so they fry in their seats.
This will obviously hinder other passengers, who did watch the safety demo and therefore have it fresh in their minds how to do this simple task.
Your air crew are very knowledgable about such matters, so perhaps you can see why they would like everyone to pay attention to the demo.