We should expect more professional courtesy and human decency for our peers than what typical application processes provide today. It help make candidates identify weaknesses and collectively improve our field.
Perhaps creativity or persistence will help us solve this difficult problem but we won’t get there if we write it off as a fact of life.
Let's suppose there were a no liability clause for employers when giving feedback, they could say we didn't hire you because we didn't like you or because you were too short, or whatever they wanted. In that case, companies could tell the truth and would have nothing to lose. However, the question would remain - what do they have to gain? To this end, if an applicant could purchase a feedback review, then companies would have something to gain as well.
I'm not saying we should do any of this, I'm just trying to sketch a scenario in which some systematic rules exist which might get both parties to be more aligned on "giving true detailed feedback".
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?
If you are a hiring manager, what are some things you have seen that you though took some initiative, but also couldn't be seen as crummy?