Why don't HR manager reply to the unapproved interviewer?

Did you mean 'Why do HR Managers not notify unsuccessful applicants?"

To make motivate for next. Because, no one is junk. Just  time, situation, and specificity makes one fit. As for example a Cricket player may not well fitted for saucer. Again, all players do not perform equally all the time. So, one time failure does not mean that future failure. Another matter, due to limited scope, or for some extra reasons , HR executive selects one, it does not mean that no others  are/were better than that applicant in that pool. It is case dependent. Thus, he/she sends such reply message to assuage. 

They’re still deciding.

We all know people who will avoid confrontation or disappointment at all costs, and hiring managers are not exempt. The thought of emailing or calling someone to tell them that they have not gotten the job is just too big of a burden to bear, especially after they’ve met and spoken to the person. So they just procrastinate on the task endlessly, or until it’s just been too long and they think,“the candidate has moved on anyway, no need to bring up bad news.” Clearly, when this is the case, the hiring manager should realize that they’re in the wrong line of work, but it still sucks.

Telling an interviewee they didn’t get the job is a tricky legal limbo to walk.

We live in a litigious society, and hiring managers want to stay on the right side of the employment law. Often times, candidates who aren’t hired will ask hiring managers for feedback on their job application performance. Why didn’t they get the job? What could they do differently in the future? Are there specific changes the employer can recommend for them? Sometimes, hiring managers will opt to not respond at all, rather than opening themselves up to potential liability through inappropriate feedback.

For example, one recruiter told us: “One reason I wouldn’t get back to someone is if I had to tell them something that they couldn’t ‘fix,’ such as their personality. If I didn’t like them, I’m not going to respond back. You don’t want to offend the person—or argue with him—so you stay quiet.”

The job might not be available after all.

The recruiting process can start and stop and be interrupted along the way. Maybe the company started interviewing external candidates but then had a couple internal people pop up. Or the funding for the position has come into question. There are a lot of “structural” reasons that a job opening might fall through that have nothing to do with your quality as a candidate.

The hiring manager isn’t the person deciding.

Sometimes individual hiring managers need to get approval from their teams, superiors, or other teams before responding to a hire, whether they’re offering them a job or not. The authorization for them to give you an answer or status update is essentially taken from them, and so they stay quiet. With different cooks in the kitchen, responding to candidates can get overlooked when they're finally able to do so.

And yes, some people really are just rude.

I’ve worked with hiring managers and human resources professionals all my life, and as with every other profession, most are responsible, upstanding, and kind people. But some are just rude and don’t care. If “you’re not going to be the person they pick, then they move on.” It sounds harsh, and it is, but unfortunately it’s also true. The silver lining? You probably wouldn’t want to work for that person anyway.

Personally, I think that employers who do this are incredibly short-sighted because it gives your company a bad name -- not just in hiring, but in general. Call it karma, but that kind of thing can come back and bite you in the you-know-what, if you ask me. What do you think?

they  dont want to disturib   in the negative