"I don't know" is not a very strange answer if you are talking to somebody at a carreer fair who is introverted. I know lots of folks who would answer like that. Once you get to know them and talk to them in a quiet non-stressful environment where they can think a little, they often open up about their real motivations.

Nobody is going to tell you the story of their grandmother with whom they shared a chemistry hobby and in whose memory they wanted to acquire deep knowledge of the field before getting a well-paying job in a different field if they are introvert and you give them the vibe of being judgemental of their life choices.