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> In my.
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.
