Question 1: Why is it grammatically incorrect if a waiter tells you to “Enjoy!”
Answer 1: It’s a useless thing to say because if the food is good you’ll enjoy it whether the waiter tells you to or not.
Answer 2: Because it might cause you to wonder whether your food or drink has been poisoned.
Answer 3: Because food is not enjoyable.
Question 2: Why is “raining cats and dogs” a dead metaphor?
(The only) answer: In the olden days, cats used to fall off roofs when it rained but they don’t do that anymore and people don’t realise that.
Challenge: If anyone can correctly guess the age and educational level of the people who provided these answers, I will personally come to your kitchen and cook you a souffle.