Thus we say: You never know, which is why. but You never know. That is why.
Understanding the Context
And goes on to explain: There is a subtle but important difference between the use of that and which in a. Why is used here as an interjection. According to Merriam-Webster: —used to express mild surprise, hesitation, approval, disapproval, or impatience <why, here's what I was looking for>. Nov 25, 2010 · Maybe I could say the question with why is a blunt question, whereas the question with how come indicates we admire that the person spoken to has such great knowledge.
