- They don't realize it's a joke. This is the most likely explanation. The person may simply not have a good sense of humor or may not be familiar with the particular joke or reference.
- They're trying to be helpful. Some people feel compelled to answer any question that is asked of them, even if it's a joke. They may think that the person asking the question is actually looking for information and they want to be helpful.
- They're trying to be funny. Sometimes people will answer a joke question seriously as a way to be funny. They may think it's clever or ironic to give a serious answer to a joke question.
Ultimately, there is no one definitive answer to this question. It's possible that a person who answers a joke question seriously is doing so for any of these reasons or even some other reason unrelated to comedy.