Why not cook？还是Why not to cook？哪个正确？

前者是正确的。
表示为什么不？
英语有两种表达：1Why don't you...?
2.Why not +V.原?
e.g.Why not go out for a walk?=Why don't you
go out for a walk?

Why don't you cook=Why not cook

确切说2个都不对。

你是想表达什么意思呢。

英语的口语是可以省略的，但是需要语境。

从这个题目来看，是 WHY DON'T YOU .... 句型

Why don't you cook? 你为什么不做饭呢。

A.We don't cook today.
B.Why not(= why don't you cook today)?

A. I am bored.
B. Why not cook ? (Why don't you cook, so that you have something to do and won't be bored.)