When I was first starting out, I found a job posting that I really wanted, so I applied and interviewed. I felt the interview went pretty well, but I got a call back that I wasn't what they were looking for. It was a small company, and the head of IT called me back to give me the bad news.
I really wanted the job, so I asked why. He told me I was too junior of a developer, and didn't understand OOP well enough. Being young and cocky as I was, I straight up told him I disagreed (looking back nearly 15 years, he was totally right). I didn't recall talking about OOP, per se, in the interview, and I had nothing to lose at this point, so I pressed the matter. I doubt it was OOP specifically that concerned him, and that it was more of a proxy for my inexperience. Either way, somehow I managed to get him to ask me about OOP over the phone. We talked a little about the vague concepts, and I have no fucking idea what happened, but he changed his mind and offered me the job. It was a great job, and I quickly learned how inexperienced I really was, but I grew a lot.
What am I getting at... in forums, you still _want_ them to participate, and come back. It is the opposite in an interview, you have essentially ended the relationship. Offering feedback starts the conversation again, and gives punks like me a chance to drag it out, disagree, and waste your time. We are just as likely to disagree with your assessment, if not more so, than to take it to heart.
Thank god he gave me another chance, I loved that job
Surprisingly enough, one of the places I least expected to get useful feedback from wasn't some small informal start-up, but a fintech giant Citadel. I already had a feeling about my weaknesses that led to failing their onsite, but their rejection email (with the feedback) helped me way more than that. Not only they pointed out those specific things I knew about already, they also pointed out a few others that I missed (that were all true) and gave me actually something useful to work with. Just wanted to express my gratitude for that, because I definitely (at least partially) attribute my improvement in those areas to that email.
In the end, it worked out for everyone. I spent 11 years there, finally leaving after we sold the company, and still keep in touch with many of them.