Why do you think they have omitted the short answers in the following examples?
A: - I'm not quite sure what happens at a clothes swapping party.
- Oh, haven't you been to one before? I thought you had.
- No, it's my first time. (No, I haven't)
B: - Why are you giving away this dress? don't you want to keep it for a special occasion?
- No, I'm bored with it these days. (No, I don't)
C: - I like the colour of this shirt. Didn't you buy one like this last month?
- No, mine was much darker. ( No, I didn't)