"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.
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.