The question comes from a self-selecting population.
This reminds me of the joke where the economist sees the $100 bill on the ground and says if it were real, someone would have grabbed it already. It assumes that the market is perfectly efficient when it is not. There are tons of reasons why someone would work at one job when they would prefer another job:
Maybe the local job market is poor, but the worker can't move.
Maybe their spouse has a medical condition that requires really good health insurance, and moving to another job would lose that coverage.
Maybe the worker has been job-hopping too much and has to stay at this job for another year or whatever.
etc., etc.
Everybody would improve something, and it can give you insight into actual day-to-day challenges.
If they say, “Nothing,” either it’s the one rare perfect workplace or they are too afraid or checked out to give a real answer.
Either way it’s informative.
(sort of like: "my one weakness is I work TOO hard!")