Why not ?常用于口语,表示建议。而且带有希望对方采纳的口吻。书面语可以是Why don't you do sth.?你为什么不做这件事呢?它和why not ?意思是一样的。希望对你有帮助!
Why not?
应该是Why not do sth = Why don't you do sth