whynotdo等于 why dont you do something.意思为为什么不做某事呢？

比如：Why not have a breakfast with us?=Why dont you have a breakfast with us?为什么不和我们一起吃早餐呢？

Why not go traveling in your spare time?=Why dont you go traveling in your spare time?为什么不在空闲时间旅游呢？。