I think the reason is that HR is similar to IT and QA - if they do their job right, it's like they're doing nothing at all.

My guess is that "Thank God HR was on top of that problem" is not a common phrase in meetings. Neither is "Wow, HR did a fantastic job creating that job posting, and every single candidate that they've brought in has been at least plausible." If they aren't braindead, you just take them for granted.

Systems like that tend to get neglected.