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.
Understanding the Context
That is why. And goes on to explain: There is a subtle but important difference between the use of that and which in a. As to why I do, I really don't know. (=.but if you want to ask why I do, I don't know.) I am going for sure.
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Key Insights
As to whether Jane will go along too, you will have to ask her yourself. English is fun, as.