It’s just extremely common to see tag questions that use contractions as opposed to their uncontracted forms.
A COCA search for , _v* * * ? (comma, verb, two words, and a question mark; note that “n’t” is considered a separate word) shows that “isn’t it?“ is much more common (5395 hits) than “is it not?” (334 hits).
Still, 334 hits is not insignificant. ...
Tag's generally use less formal language as a standard because tags themselves are a less formal way of speaking.
You could use the full form if you want, but the tone changes and makes the sentence more emphatic. Sometimes this change in tone is what you're intending.
You knew you were coming home late, didn't you?
... sounds less severe ...
I believe the child is just misinterpreting the question a bit. The detective asked "why", but the child interpreted this as meaning "under what circumstances".
Or he could just be phrasing the answer poorly, meaning "Because I have to go."
All these concepts are easily conflated, especially by a child in distress.
In this question, the answer is the subject.
Questions often reverse the typical syntactic order of noun then verb. Subject-auxillary inversion is frequently employed in English to form questions.
The question at hand can be dissected and reassembled as:
The answer is what?
Had it been phrased in this more awkward but grammatically correct manner, ...