Participation is fun
Because they try to be useful but they realise they don't have an answer but they still feel the urge to participate so they just say they don't know...
They honestly don't know.
Because sometimes it's the only honest answer to a sincere question. From people who I respect, 'I don't know' IS an answer that is often helpful.
Maybe you made them stop and think about something and at that moment they realized.....they don't know.
I don't know.