You ultimately open yourself up to more problems than solutions as a hiring manager. If someone said “Why didn’t I get the job” and I say “your attitude and demeanor was poor” they could come back and say “Oh well I suffer from Bipolar disorder, you’re discriminating against me and now I’m going to file a lawsuit”. This could be more problematic in the litigious states of America, but it’s common practice here when declining employees to not provide reasons.
Also, keep in mind I often go through upwards of 200-300 resumes per candidate and sometimes up to half dozen interviews. Giving each candidate feedback on why they were a no would consume more hours then are in a day.