My question is: is there flexibility in how one can punctuate the phrase "Why not?"
The answer may seem obvious at first...it is a question after all. However, it's also a common idiom, and I am wondering if that offers a choice in how it's punctuated. I became curious about this after seeing dictionaries, including Cambridge and Collins, using the phrase in example sentences sans the question mark:
"Do you want Italian food tonight?" "Sure, why not."
I said we were, too, so he said why not let him take us to dinner somewhere and then all come together.
The second example sounds especially wrong, and as if it's missing quotation marks. Of course, most dictionary examples of the phrase in a sentence included question marks. So are the provided examples above errors?
Edit: The first comment suggesting this is a possible duplicate links to a statement, not a question.