I don't know why, but it seems to me that Bob would sound a bit strange if he said, "Why is it that you have to get going?" in that situation. Thus we say: You never know, which is why. but You never know. That is why. And goes on to explain: There is a subtle but important difference between the use of that and which in a sentence,. Good explanation of why it's optional in this case, although I'm not convinced that reason is the only reasonable antecedent of why. For example, the explanation why is a common usage, and I don't.

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