A lot of the time it’s because:

1. They haven’t had time to think about it.
2. It would reveal a flaw in their logic.
3. It would reveal their ignorance.
4. They are unwilling or unable to invest the effort.
5. They think you should put in the effort to figure it out.
6. They have been programmed with some social expectation they are projecting.
7. They’ve been hurt in the past by someone demeaning them for their own lack of understanding.