so why are hrs not bothered to send an email if a candidate is rejected for some particular reason after the final round of the interview?

Nobody likes to tell no to someone. It's just a human/Indian mentality. Rejecting someone takes cojones.

They want to preserve optionality.

In worst case they fuck up with new candidate they want to be able to come to you.

Option to consider you, if the alternative arrangement fails.

It does not work at scale. People turn hostile.

Because its unnecessary. at lower end of hierarchy , people are just resources. Demand supply economics is not in our favour. Imagine your normal shopping week end or a high value purchase like car, we visit multiple shops check prices, and then you finalize the deal. Once done, we dont call others to inform them . Same story.

HR also are busy with theirs rangolis/.... so no time.

They enjoy it! Sadistic but true