Apart of the already named reasons (mainly legal): I once actually did it, because I LIKED the applicant. The result was simply lots of follow up mails on how and why I am mistaken. Wouldn't do this again.
The result was a lot of candidates trying to prove me wrong with relentless e-mails and phone calls. One of them even went on Glassdoor and other websites to write long reviews about how we misunderstood him in the interview. This was the same guy who didn’t even try to answer about half of the interview questions.
I stopped giving any feedback after that. Wish I could help candidates, but it’s not worth the risk of dealing with more angry candidates viewing it as an opening to argue.
Related: reasons you might explain as little as possible when firing someone, or when breaking up with someone.
I've done the same thing with the same result. It became an argument/pissing match that I just ended up ghosting.