IIRC, they do that because disclosing the reason could open the company up to legal liability if the reason isn't accurate or well documented enough to hold up in court in defamation case.
"Guilt" in the court of public opinion is a real thing and this industry is smaller than most people realize. You may try to get another job only to find the same people from your previous job are at the new one, know people there, or that word has just gotten around about you whether online and searchable or through word of mouth. Or maybe someone calls a previous employer and they mention why you were let go?
All for something that was false?
Would it count as slander/libel in this case? You could make the argument that it led to lost earnings or whatever too.