Like when people ask you if you have a lighter. "Sorry, I don't," goes a lot farther than "No, I don't smoke."

They're not asking you for your convictions regarding alcohol / tobacco, they're asking you if want some / have a lighter they can use.

But people don't tend to say "Why don't you have a lighter?" when you say "Sorry, I don't." in the same way that they query why you don't want a drink. I endured this for years; it gets tedious after a while.

That happens to you? Never happens to me. Either case.

At any rate, you can answer why when they ask you why.

No, you'd get "You don't want a drink? Why not?" as I experienced many times. Hence the clarification.