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.
Feb 12, 2018 · In the sentence "Why is this here?", is "why" an adverb? What part of speech is "why?" I think it modifies the verb "is", so I think it is an adverb.
Dec 4, 2018 · For why' can be idiomatic in certain contexts, but it sounds rather old-fashioned. Googling 'for why' (in quotes) I discovered that there was a single word 'forwhy' in Middle English.
