>> It help make candidates identify weaknesses and collectively improve our field.
Actually, and this will sound wrong, but it won't.
Let's say the worker pool is 1000 candidates. If I give them sll advice, and they take it, I still have 1000 candidates to choose from. It's just harder since they're all better.
On the other hand the resume that contains spelling errors is easy to just discard (low attention to detail.) I don't want someone else to gave told them, I'm trying to find people who have attention to detail without being told.
Failing that I'm looking for people with some initiative. Perhaps someone who has recruited friends, family or even a service to run mock interviews and provide feedback.
Of course resumes are terrible starting points for job applications. You send out thousands, I get thousands. How might you take that knowledge to better stand out from the crowd?
Here's my answer to the original poster; we don't give feedback because of the legal and incentive reasons others have highlighted.
Recognizing this fact, what then is your next step? Instead of being a passive participant in the process, what might you do to stand out from the crowd? What might you do differently to the herd?