But do companies explicitly say that, though? From my experience working at a YC startup, the company just used to send out a canned rejection email, irrespective of whether the candidate was rejected because of a bad interview or because someone thought they'd be a pain to work with. The startup I work at once had a staff/lead engineer from twitter interview, and while some loved him, some didn't and he was rejected.

A reasonably professional person will follow up with rejection emails in order to determine what was lacking so they can work on it. Companies who reject people should be ready to give useful feedback that people can use to improve their chances moving forward.

And, unfortunately, it is in the best interest of a reasonably professional company to decline to comment on a rejection.

Indeed, it is frustrating. It may give the impression that you are defective in some way (to improve yourself), but doesn't necessarily mean that. As the job candidate, I have declined offers because of culture fit. I have also left some companies due to culture fit. It's really a two-way street.

In a lot of cases, I think it's better to get rejected up front than to get hired on only to realize later on that you've landed in a bad situation. To me if a company wants to reject you based on culture fit, many times it's a blessing in disguise.