Thus we say: You never know, which is why but you never know. That is why and goes on to explain:

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. 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. There is no recorded reason why doe, except there was, and is, a range of others like roe. So it may have been a set of names that all rhymed and that law students could remember. Or it could be that.

So it may have been a set of names that all rhymed and that law students could remember. Or it could be that.