why not doing还是why not do?

why not 后面用DO来 表示说话人的提议，比如：
Why not go climbing the mountain today? The weather is so fine.
今天去爬山怎么样？天气真好

why not也可以单独使用，表示说话人认同对方的提议。

如：---let's go to the party and have fun.
---Why not?
－－我们一起去聚会玩个开心吧
－－为什么不呢？（意思就是：真是个好主意）

How about doing...?
What about doing...?
Why not do...?

why not do something

why not do sth 是对的。=== why don't you do sth 是一个建议的句式。
明了吧

why not do 呀~
eg。why not go to the cinema

why not do sth
你八成是记错了