A friend of mine did a PhD in computer science solely and specifically so he could prove to himself that he could do a sustained, multi-year solo project and complete it. He then got a job he totally would've gotten 4 years earlier as well, with just the MSc. When asked (eg by recruiters/interviewers), he always said "because I wanted to prove to myself that I could do it" and personally I find that a very satisfying answer.
All I'm saying is, not everyone doing a PhD and then leaving academia was accidentally climbing up the wrong ladder. Some did it on purpose, just to admire the view.
"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.
All I'm saying is, not everyone doing a PhD and then leaving academia was accidentally climbing up the wrong ladder. Some did it on purpose, just to admire the view.