Why Do I Get Light Headed When I Stand Up - Health Inspiration

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. Why don't you give me that book? Why don't you go to the store and get some more milk?

Why don't you make me a sandwich? In these examples, the speaker is clearly not asking for the reason the listener is not doing the action in question.