I'm pretty sure that if a company cuts off all contact after an interview, there is at least one problem on their end that I cannot fix, no matter how hard I try.
Believe me, I have tried to find out why they vanished from the face of my planet. But there's only so much effort I can put into throwing politeness and courtesy into the void of unimaginable rudeness.
You have to do mock interviews to get real feedback. Real interviewers are so absolutely terrified of telling you even one thing that might help with your next interview that I have never heard anything, good or bad, about myself in the wake of a real interview.
This is not an occasional problem. This is epidemic in every city I have tried looking for full time employment, to the point where I am actually shocked--shocked--if someone follows up with me after an interview when they do not want to extend an offer.
The only reason I can think of for this is that someone out there is actively counseling companies and HR managers to conduct their interviews in this manner. Perhaps it is to cut costs. Perhaps it is to minimize liability exposure to workplace discrimination laws. Perhaps I am on some sort of blacklist. Maybe something on my credit report is bad. There is literally no way for me to find out, because I am already blackholed by the time I might get a chance to ask, and I have no expectation that they would answer honestly if they would even answer at all.