- Whoever submitted the question very likely doesn't understand the question well enough to answer it themselves, so any feedback is not going to get a reasonable answer.

- The amount of time to check whether an answer is correct is non-zero. If you could somehow know that the answer was written by a human that ensures that the effort on the part of the answerer was non-trivial, and the "proof of work" infuses the answer with a minimum amount of trust which is absent in the case of a generated answer. Compare to a spam email: you wouldn't read all of the emails in your spam box thoroughly to determine if any of them contained a nugget of truth. You'd assume ill intent, and treat the contents accordingly.