Why not add the verb prototype after why not, Why not do this? ...
For example, why not have some fish and rice? Why not eat some fish and rice?
Why not means why not, and why means "why", which can be used as interrogative adverbs, relative adverbs and connecting adverbs to guide noun clauses.
Why not? The synonym is why do not you do, in which why, as an interrogative adverb, leads special interrogative sentences or omitted special interrogative sentences to express doubts, indicating "why", followed by an infinitive without to, indicating "why …", indicating that an action is unnecessary or meaningless.
Example: Why don't you do what you did to your mother? Isn't it the same when you are with your mother?
Four usages of why not in spoken English;
1. Agree or agree: Yes; Yes; Why not?
Can I go shopping with you? Can I go shopping with you?
Why not? Of course.
2. Express persuasion, suggestion or command: How about ...; Why not? ...
Why don't you ask me for help? Why don't you ask me for help?
3, used to ask the reason: why; Why not? ...
He is not going to see her off at the station. He is not going to see her off at the station.
Why not? Why?
4. Show that you stick to your point of view, attitude or practice: Why not?
Do you really want to reward them? Do you really want to reward them?
Yes, why not? Yes, why not?