As someone who has dealt with HR departments since before the time they started to adopt this new role you mention, I can testify you have hit the nail on the head.

HR not only makes it hard on the candidate, and the employee, but on the hiring manager, as well. I've worn all three hats.

One of the major roles of HR is to turn the hiring/firing/personnel aspects into a procedure that can be manipulated without it appearing to be directed at an individual (which is illegal).
The original justification for this was a good one, it kept personalities out of it and kept the process fair.

But the system has now calcified to the point where it operates with a mind of its own, primarily for the benefit of those who run it. And the clever executive familiar with its operation can use it as weapon to get what he wants, whether it is good for the company or not.

Except at the very lowest levels, its STILL a good ole boy club.