他为什么不知道？

What's the reason he don't know it?

youdao

为什么？你为什么不知道啊，爸爸？

Why? Why don't you know, papa?

youdao

我不知道为什么。

I don't know why.

《柯林斯英汉双解大词典》

And I don't know why, but since time immemorial that is the way computer scientists have drawn trees.

叶子是在最下面的，我不知道为什么,但从记事以来他们就是这么画树的,这也许就是为什么我们。

麻省理工公开课 - 计算机科学及编程导论课程节选

But for me, I like to look at something and just say, "I like this. I don't really know why though."

但对我来说，我喜欢看着某个东西，然后说，“我喜欢这个。但我也不知道为什么。”

关于现代美术 - SpeakingMax英语口语达人

If you don't know why you're about to eat cake like that you will soon, but you know this.

如果你现在还不知道为什么将要吃到那样的蛋糕,很快就会知道的，你清楚这一点。

哈佛公开课 - 计算机科学课程节选